The swimming and buoyancy of the gelatinous pteropod,
Cymbulia peroni, was observed and recorded on videotape from a submersible in the western Mediterranean. When not swimming Cymbulia peronihangs neutrally buoyant, with its parapodia outstretched and motionless. Two modes of swimming were observed, slow swimming and rapid or ‘escape’ swimming. Slow swimming predominated, at speeds between 2 and 11 cm s?1. Most slow swimming was directed vertically up or down. Slow swimming is probably used for vertical migration and the avoidance of turbulence. Turbulence frequently occurs both at epipelagic and mesopelagic depths in the Mediterranean. Escape swimming was more rapid, and speeds up to 40 cm s?1 were measured. Differences between slow and escape swimming appear to be quantitative rather than qualitative, in contrast to other cymbuliids. Cymbulia peroniis an active and vigorous swimmer, clearly capable of vertical migration, and probably attains speeds greater than the highest measured in this study. Its behaviour in the field differs dramatically from its behaviour in the laboratory.