Résumé : Crassolide, a monocyclic diterpene isolated and purified from the soft coral Lobophytum crassum, inhibited the cell cleavage of sea urchin eggs without affecting fertilization. The effect was observed with concentrations above 2 × 10-5 m in egg suspensions. Addition of crassolide between 5 and 40 min post-fertilization totally blocked the first cleavage, which in the control occurs 1 hr after fertilization. When added between 50 and 60 min post-fertilization, crassolide produced polynucleated cells in embryos. Crassolide did not affect the egg permeability to Na+ and Ca2+, but caused an increase of 0.2 units in the intracellular pH of fertilized eggs coupled with a proton efflux. Crassolide, which does not affect Ca2+ influx or permeability at the level of storage in reticular vesicles, could be used as a negative control when analysing calcium changes in short-term toxicological studies. The relationship between the pH increase and the cell cleavage needs further investigation. © 1991.