We have examined the arrangements of the thick and thin filaments of the smooth mus-cle of the adductor of the bivalve Cristaria plicata, Leach, in the catch state induced byacetylcholine, both in ultrathin sections and in separate filaments. The thick filaments areheavily bent and entangled and the thin filaments irregularly arranged. One thin filamentmay be attached to several thick filaments. When the muscle is in the relaxed state, the twokinds of filaments are seen to be in Parallel. However, no difference could be observed in theperiodic strusture of the paramyosin core of the thick filaments in the relaxed state or in catch.Both cross-striated and checkerboard types coexist. It seems that phase transition is not di-rectly related to catch. We propose that the disordered arrangements of the thick and thinfilaments and the formation of three-dimensional anastomosed networks are the structural basisfor catch, with a consequent loss of the capability of directional sliding of the filaments andrequiri We have examined the arrangements of the thick and thin filaments of the smooth mus-cle of the adductor of the bivalve Cristaria plicata, Leach, in the catch state induced by acetylcholine, both in ultrathin sections and in separate filaments. The thick filaments are heavily bent and entangled and the thin filaments irregularly arranged. When be attached to several thick filaments. When the muscle is in the relaxed state, the twokinds of filaments are seen to be in parallel. However, no difference could be observed in theperiodic strusture of the paramyosin core of the thick filaments in the relaxed state or in catch. Both cross-striated and checkerboard types coexist. It seems that phase transition is not di-rectly related to catch. We propose that the disordered arrangements of the thick and thin films and the formation of three-dimensional anastomosed networks are the structural basis for catch, with a consequent loss of the capability of directional sliding of the fi laments andrequiri