Sea urchin embryos of the species Paracentrotus lividus were treated continuously with different concentrations of all-trans retinoic acid (RA) or dimethylsulfoxide (DMSO) at different developmental stages. A delay in embryonic development was observed when embryos were cultured in the presence of 2x10−5 M RA, between 1 and 12 hours of development. Hence, at 48 hours of development, while control embryos had reached the pluteus stage, RA-treated embryos were at the prism stage. At 72 hours of development RA-treated embryos recovered and continued normal development reaching the pluteus stage. No effect was observed when treatment was performed before 1 hour or after 12 hours of developmet. DMSO treatment had no effect on normal sea urchin embryo development, although we observed that pigment cells, clearly visible at the pluteus stage, become visible earlier with respect to control embryos. This report confirms the advantages that the sea urchin embryo offers for the study of problems in cellular and developmental biology.