For the first time, effective treatments using cytochalasin B were developed to induce triploidy in the European clam, Ruditapes decussatus (L.). The percentage of triploid embrios was assessed by karyological or image analysis. Two treatments (O.5 or 1 mg of cytochasin B (CB per ml of dimethylsulfoxide in 1 l of sea water) were applied at different times after fertilization (10 to 25 mn), for two different periods (15 and 20 min). Best results were obtained for a CB concentration of 1 mg/l. When treatment was applied 15 min after fertilization for a 20 min period, 94% and 95% of triploid embryos were obtained in two repeated experiments. At metamorphosis, the treated larvae appeared to be no smaller than the control larvae in all experiments. Thowever, in general, significantly higher mortalities for CB treatd batches weres found when compared with the untreated batch.