The biological activity of a methanolic extract of the introduced species Caulerpa taxifolia and caulerpenyne (main secondary metabolite of the alga) was tested on embryogenesis, pluteus and metamorphosis stages of the sea urchin Paracentrotus lividus. The bioassays revealed different levels of toxicity according to the egg and larval development stages. Larval development was the most sensitive phase, followed by gastrulation. Methanolic extracts at a concentration of 1.0 mu g ml(-1) and caulerpenyne at a concentration of 100 ng ml(-1) (300 nM) tested on young larvae (4-arm stage) cause death within four hours. The same concentrations of methanolic extract, tested on eggs, blocked cleavage up to early blastula stage, whereas a double concentration of caulerpenyne was necessary to obtain the same results. After exposure to 0.2 and 0.6 mu g ml(-1) of methanolic extract, larvae at 8-arm stage (when echinus rudiment develops inside the body) displayed abnormal development and high mortality. When larvae became competent for metamorphosis, the presence of 4.0 mu g ml(-1) of methanolic extract and 400 ng ml(-1) of caulerpenyne did not alter the success of this process. Therefore, the methanolic extract expressed as caulerpenyne equivalent is more active than pure caulerpenyne, suggesting that other secondary metabolites in the alga are also responsible for the toxicity.