Recent molecular work has revealed a large diversity of myosin heavy chain (MyHC) gene variants in the abdominal musculature of gammarid amphipods. An unusual truncated MyHC transcript from the loop 1 region (Variant A3) was consistently observed in multiple species and populations. The current study aimed to determine whether this MyHC variant is specific to a particular muscle fibre type, as a change in net charge to the loop 1 region of Variant A3 could be functionally significant. The localisation of different fibre types within the abdominal musculature of several gammarid species revealed that the deep flexor and extensor muscles are fast-twitch muscle fibres. The dorsal superficial muscles were identified as slow fibres and the muscles extrinsic to the pleopods were identified as intermediate fibres. Amplification of loop 1 region mRNA from isolated superficial extensor and deep flexor muscles, and subsequent liquid chromatography and sequence analysis revealed that Variant A3 was the primary MyHC variant in slow muscles, and the conserved A1 sequence was the primary variant in fast muscles. The specific role of Variant A3 in the slow muscles remains to be investigated.