In the sea squirt Phallusia mammillata, fertilization occurs externally in the seawater surrounding the adult. The impact of mercuric ions, one of the most toxic environmental pollutants, was investigated on the functioning of the ionic channels of unfertilized or fertilized egg membrane and on the associated egg morphological events occurring at fertilization. Hg 2+ ions in the submicromolar range affected in a dose-dependent manner (range 0.0625 to 2 µM) the functioning of voltage-dependent Na + and Ca 2+ channels pre-existing in the unfertilized oocyte membrane. These channels are also sequentially involved in the first step of the egg electrical response to fertilization (i.e., the fertilization potential) and were impaired by Hg 2+ . Mercuric ions also strongly inhibited the further operating sperm-dependent channels of this fertilization potential. The two highest concentrations used (1 and 2 µM) greatly reduced the number of eggs that responded electrically and, as a consequence, diminished the percentage of fertilized eggs that resumed meiosis. Also as a consequence of the impairment by Hg 2+ ions of the egg electrical response to fertilization, the electrical block to polyspermy became less efficient and the proportion of polyspermic eggs increased. Finally, the percentage of eggs containing a male pronucleus over the total number of sperm-penetrated eggs statistically diminished in a dose-dependent manner under the effect of Hg 2+ ions.