Pesticides are of ample worldwide use. At present there is a growing concern about seawater contamination by these chemicals. Developing aquatic organisms are particularly at risk. For this reason, early development of Octopus mimus embryos under exposure to commercial Parathion was examined. Concentrations over 0.4 mM of the pesticide altered formation of the embryonic disc at the blastula stage, resulting in abnormal gastrulation and arrest of further development. This toxic effect may be due to the blockade of DNA synthesis elicited by organophosphoric pesticides causing diminished number of blastodermic cells at the time of epiboly, thus hindering the start of invagination. Seawater contamination seems, therefore, of relevance for survival of exposed aquatic species.