The Kaloe population of Zources viviparus was sampled in two successive winters for males, pregnant and non-pregnant females, and the EstlIl genotypes of the fish, including motheroffspring combinations, were scored. The observations were submitted to a selection component analysis which revealed significant evidence of a change in genotypic structure that occurs in the period between the foetal and the adult stage. It is argued that this genotypic distortion, which leads to a gene frequency shift, is most likely due to zygotic selection through either differential survival or differential emigration. The observations indicate that the zygotic gene frequency shift is counteracted by either male sexual or male gametic selection, but there is no statistically significant proof of this contention.

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