The effects of cytochalasin B on fertilization and ploidy in Crassostrea gigas were examined in a controlled experiment. This enabled us to better understand why variable ploidy results have been found with the retention of polar bodies in previous studies on bivalve mollusks, and what the interaction with fertilization may be. By looking at the early development of Crassostrea gigas embryos with sequential samples stained using Hoechst 33258, a fluorescent DNA-specific dye, we compared normal evolution of fertilized eggs with that obtained with cytochalasin B treatment. In treated eggs, embryos with both three and four pronuclei were found after the second meiotic division. When embryos had three pronuclei, these were identified as the female pronucleus plus two chromatin groups arising from the retention of the two polar bodies. The fourth pronucleus present in some embryos was the male pronucleus. The absence of the male pronucleus in the embryos with only three pronuclei was interpreted as gynogenetic development due to the inhibitory effects of cytochalasin B on penetration of the sperm into the eggs. This result explains the findings of several previous studies where different ploidies resulted from double polar body inhibition in bivalve species. Implications of such treatments for post-embryogenic development are discussed.