SUMMARY The anterior byssal retractor muscle (ABRM) of the bivalve Mytilus edulis shows a prolonged tonic contraction, called the catch state. To investigate the catch mechanism, details of which still remain obscure, we studied the mechanical responses of ABRM fibres to quick increases in load applied during maximum active isometric force (P0)generation and during the catch state. The mechanical response consisted of three components: (1) initial extension of the series elastic component (SEC),(2) early isotonic fibre lengthening with decreasing velocity, and (3) late steady isotonic fibre lengthening. The ABRM fibres could bear extremely large loads up to 10–15P0 for more than 30–60 s,while being lengthened extremely slowly. If, on the other hand, quick increases in load were applied during the early isometric force development,the ABRM fibres were lengthened rapidly (`give') under loads of 1.5–2P0. These findings might possibly be explained by two independent systems acting in parallel with each other; one is the actomyosin system producing active shortening and active force generation,while the other is the load-bearing system responsible for the extremely marked load-bearing ability as well as the maintenance of the catch state.