Thick filaments from annelid muscle were observed by positive and negative staining of the glycerinated sample, which was homogenized in a relaxing medium containing ATP. The preservation of myosin on the filament surface was strictly related to very high concentration of ATP (12 m M .). The 144 A periodicity was scarcely visible. When the ATP in the homogenate was at low concentration (less than 3 m M ), the underlyng periodic structure of paramyosin appeared. Purified paramyosin cores (at low pH and high ionic strength) presented a striation with a 144 A pitch. The Bear-Selby net pattern was also present, even in a small number of filaments. Paramyosin was extracted and purified from fresh muscles of Lumbricus terrestris and Sipunculus nudus . The purified protein exibited a single peak in the ultracentrifuge, and the intrinsic sedimentation constant was 3.29 S for Lumbricus and 3.19 S for Sipunculus . After dialysis with divalent cations, paracrystals were obtained, most of which showed a 144 A periodicity; sometimes, paracrystals with a different band pattern appeared. In all tactoids observed a 48 A subperiod was evident, as in the native isolated cores.