Food preference and growth were investigated in the ascoglossan opisthobranchs Oxynoe azuro- punctata Jensen, 1980 and Elysia cauze Marcus, 1957 fed on four species of Caulerpa (Chlorophyta, Siphonales). No food preference was found if animal size was disregarded. However, animals preferring Caulerpa species with narrow filaments were significantly smaller than those preferring Caulerpa species with larger filaments. Growth in O. azuropunctata >4 mm was significantly higher when feeding on Caulerpa sertularioides (Gmelin) Howe than when feeding on C. paspaloides (Bory) Greville. Growth in animals ≦4 mm was not significantly different on the two food algae. Juvenile E. cauze grew significantly faster on C. verticillata J. Agardh than on C. ash- meadii Harvey. These differences in preference and growth are attributed to more efficient feeding on algae with suitable filament diameter relative to animal size.