Abstract.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 embryos was assessed by karyological or image analysis. Two treatments (0·5 or 1 mg of cytochatasin B (CB) per ml of dimethyisulfoxide in 1 litre of sea water) were applied at different times after fertilization (10 to 25min), for two different periods (15 and 20min). Best results were obtained for a CB concentration of 1 mg/1. When treatment was applied 15min after fertilization for a 20min period, 94 and 95 of triploid embyros were obtained in two repeated experiments. At metamorphosis, the treated larvae appeared to be no smaller than the control larvae in all experiments. However, in general, significantly higher mortalities for CB‐treated batches were found when compared with the untreated bat
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