The effect of retinoic acid [RA] on the morphogenesis of freshwater sponges (Spongilla lacustris, Ephydatia fluviatilis, Ephydatia muelleri) was studied. At a concentration of 2 μM, RA had no overt effect on pinacocytes and choanocytes but had already altered mesohyl cell movement and adhesion after 1 h treatment. It interfered with morphogenesis of sponges developing both from larvae and gemmules, inhibiting the formation of the aquiferous system, and promoting formation of reduction bodies. The same was observed in treatment of functional sponges with RA. We conclude that RA induces modifications in mesohyl cell adhesion and interaction, analogous to that observed in induction of reduction bodies formation or gemmulogenesis.