Taxonomic studies involving only adult forms of organisms may not necessarily provide complete information about differentiation of species or about the evolutionary relationships between species grouped in higher taxa. The taxonomic importance of embryonic or larval morphology has been limited only by the lack of detailed comparative morphological work on development. Fretter (1967) has shown that larval shell characteristics are reliable for taxonomic identification of some British prosobranchs. Ostergaard (1950) proposed using the structure of egg masses, larval shell type and developmental characteristics to confirm adult taxonomic position. In recent literature reviews of the larval biology of opisthobranch, it has become apparent that a considerable amount of plasticity occurs with respect to patterns for development. Often, closely related species have radically different developmental types. Even different populations of a single species may exhibit different development. Often the ecology of the adults of closely related species is also different. However, it is not possible to make meaningful correlations between the biology of the adults and the larvae unless additional ecological and developmental information is available. There is a clear need for studies of larval development and the ways in which larval characteristics and embryology are correlated with adult biology in marine organisms. Aspects of the larval development and adult ecology of Phyllaplysia taylori Dall, are reported in this study.