The first parade of the Battalion took place in the Colston Hall on the 21st September when the Lord Mayor of Bristol headed the inspection and gave a speech to the massed ranks of Glosters. After the review the Battalion paraded by companies, in Colston Avenue, and marched from there through Baldwin Street, Bridge Street, Union Street, Lower Maudlin Street, Maudlin Street, Park Row, and Queen's Road to the Victoria Rooms, returning via Park Street to the Colston Hall. On the return journey, about thirty old Crimea and Indian Mutiny Veterans were assembled on the corner of College Green together with their standard bearer, Mr. Singleton, who held aloft a Union Jack and alongside it was a broad banner bearing the words: 'We have done our duty; come and do yours.' As the Battalion marched down Park Street, the banner caught the troops' eye, and as one, they saluted the little group to the delight of the cheering spectators.

The War Office announced that in future 'Bristol's Own' was to be known as the 12th (Imperial Service) Battalion, The Gloucestershire Regiment. With the Battalion now installed in their Ashton Gate headquarters, training began in earnest, although it was short of equipment (especially rifles), hence the never ending ritual of drilling, marching and manoeuverings. However so much progress had been made that on the 14th November the War Office considered it time for a technical inspection by Major-General Dickson which was to put the men's newly won proficiency to the test, which reports indicate they came through successfully.

The Battalion was originally to comprise 1,100 men, but on the 30th November the War Office issued instructions to raise another company of 250 men, and by 1st March 1915, the total number of enlisted amounted to 1,322 men, a fact that shows the popularity of the unit which proved a valuable stepping-stone to promotion for many of its more promising young men. The number of recruits far exceeded expectations, so much so that another company (making the sixth in all) was raised. Although the majority of the recruits were natives of Bristol, many came from other towns and counties in the West. Some of the neighbouring places provided many recruits; for instance Weston-super-Mare sent a batch of eighty recruits.

About a month later the troops received their uniforms, and on the 12th December the Lord Mayor and Sheriff witnessed the Battalion's first formal 'march past' in Ashton Park in full uniform and equipment. Recalling the initial shortage of arms and ammunition, the Battalion was generously provided with 80,000 rounds of ammunition by the local inhabitants. After the inspection came a tour of the headquarters by Civic dignatories, and later a parade through the city was carried out, with citizens turning out in their thousands to watch the march past.

During the winter months the Battalion spent its days in drilling, route marching, field work, rifle and bayonet practice, and physical exercise. The date of the 23rd June was fixed for the Battalion's departure for their summer camp at Wensleydale, in Yorkshire, where they were assigned to the 95th Brigade, in the 32nd Division. By August the Battalion had moved to Salisbury Plain for final training. On the 21st November the Battalion left their camp at Codford, near Westbury, Wiltshire and entrained at Wylye Railway Station for the port of Folkestone and the subsequent crossing to Boulogne, France, under the command of Lieut-Colonel M. Archer-Slee, DSO, MP. Arriving at 11.15am the Battalion comprising 31 officers and 988 men, moved straight on board the waiting transport 'Stranraer'. Boulogne was reached at 20.00 and they proceeded to Ostrohore Rest Camp. The next day their interpretor Joseph Storch arrived at the camp. On the 23rd November they marched to Gare and entrained. They arrived at Longpre at 7.30am and detrained, and then marched to Buigny L'Abbe. On the 27th they paraded and moved to L'Etoile, about 10 miles, and the following day marched the 16 miles to Bertangles where they remained until the end of the month.

On 1st December the Battalion moved to billets at La Houssoye and the next day went on to Sailly Lorrette. On 3rd December the 12th were divided into two, with 17 officers and 486 men of 'A' and 'B' companies moving into billets at Suzanne. For the next five days the two companies were in A2 sector attached to the 2nd Inniskillin Fusiliers and the Duke of Cornwall's Light Infantry for instruction. The 8th December saw the Battalion's first casualty of the War when Private F.M. Vowles was wounded in the hand. On Friday 10th December Private Alfred Lewis of 'C' Company became its first fatality when a shelter fell on him. Private W.G. Collins was killed later the same day by a rifle bullet. The men of 'C' and 'D' company were relieved by the 1st Manchester and DCLI and returned to billets at Suzanne. Many of the men were by now exhausted and lots equipment was lost on leaving the trenches, which were in a dreadful state. Private F.E. Cook from 'D' company was mortally wounded in the side and died the same day.

On 12th December, 2 days after being killed, the bodies of Privates Cook and Collins were buried at Suzanne and Maricourt respectively. On the 13th the battalion was back in the trenches. The battalion remained at Maricourt until the end of the month. On the 28th the battalion was assigned to the 14th Brigade, in the 5th Division. The first week of 1916 saw the battalion holding trenches in sector A3. On the 9th the battalion moved to Sailly Lorette and on th 10th to Allonville into the 5th Division Rest Area. On the 22nd January a draft of 20 men arrived.

By July the battalion was part of 95th Brigade, 5th Division. They left Candas and arrived at Ivergny on the 14th July. After moves to Puchevillers (15th), Bresle (16th), Becordel-Becourt (17th) and Mametz (19th), they went into the support line Caterpillar Valley on the 20th July. After a short period at Longueval they were relieved to Pommiers Redoubt on the 26th. On the 28th July they moved into the front line and 'B' and 'C' companies attacked at 3.30pm. By the next day they had taken the German line north of Duke Street and then withdrew to Pommiers Redoubt. On 1st August they moved to the rest camp at Dernacourt and on the 5th entrained at Mericourt for Airaines, from there they marched to Vergies. On the 25th August they moved to Dernacourt and then to Citadel Camp the next day. On the 29th they moved into reserve trenches near Talus Boise, Bronfay Farm on the 29th and attacked the German line from Wedge Wood to Guillemont, taking it but sustaining 328 casualties. On the 5th September they moved to Leuze Wood. On the 6th moved back to Happy Valley, and to Ville-sur-Ancre on the 9th. On the 18th they moved to the front line north of Bouleaux Wood. On the 20th relieved to bivouacs near Ginchy. On the 22nd they moved into support lines north-west of Wedge Wood and to assembly trenches near Ginchy on the 24th.

25th September they attacked Morval (with the 2nd King's Own Scottish Borderers) and took their objectives. On the 26th they moved to Oxford Copse, Citadel Camp (27th). They entrained at Grove Town on the 29th and then marched from Longpre to Sorel. On 1st October they entrained at Pont-Remy for Chocques and Bethune sector.