All-trans retinoic acid is known as a teratogen in vertebrate development. To study whether molluscan morphogenesis is sensitive to retinoic acid, the development of Lymnaea stagnalis, Physa fontinalis and Bithynia tentaculata was examined after treatment with retinoic acid. Low concentrations retinoic acid (10–7M) specifically affected eye formation in each of these species. In Lymnaea, it was shown that 10–6M retinoic acid resulted in a wider spectrum of deficiencies, including eye defects, arrested development and shell deformations. Pulse treatments revealed that embryos were most sensitive during gastrulation. Soon after gastrulation the embryos had lost their sensitivity to retinoic acid, which indicates that the observed defects do not result from a general non-specific toxic effect on cells. Even though molluscan development differs in numerous respects from vertebrate development, the present results suggest that molluscs share common features with vertebrates in morphogenetic processes that operate in early development.