Fertilized eggs of the mussel Mytilus galloprovincialis were exposed to cytochalasin B (CB, 1mg/l) from 7until 35minutes after insemination to inhibit both polar body I and II formation. Cytological observations revealed that many of the CB-treated eggs contained five pronuclei. DNA microfluorometry of trochophore larvae from CB-treated eggs indicated ploidy levels from diploid to decaploid, with 18% classified as tetraploid and 12.4% as pentaploid. Survival from the trochophore stage to day 20 was 8.2% for the CB group compared to 48.8% for the control group. One month after initial settlement, DNA microfluorometric analysis of 29 individual spat from the CB group showed that 24.1, 58.7, and 17.2% were diploid, triploid, and tetraploid, respectively. Previously, induced tetraploidy has only been found in embryos and larvae. The discovery of tetraploid spat indicates a potential for tetraploid bivalve production.