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Ktly WYDY~S: Heritability of sex ratio; Oplqwtrochu luhronicu; polychaete worm; sex determination; sex ratio. AbstractOphryotrochu luhronica is a gonochoristic polychaete worm whose sex detcrmining mechanism and sex ratio control are supposed to be polygenic. From a lab population, whose sex ratio (i.e., proportion of males) was 0.5, the estimate of sex ratio heritability by offspring-father regression was 0.54 f 0. I5 and by offspringmother regression was not significantly difrerent from 0. Estimate of sex ratio repeatability between successive broods of a pair was 0.64 & 0.33. Since female parents do not contribute in any way to the variability of sex ratio, sex ratio variation seems to be largely a paternal character. On the basis of these estimates we advance the hypothesis that in this species sex is determined by a multilocus genetic system, allowing the combined effects of a female major sex gene (which could give rise to a form of female heterogamety) and masculinizing modifiers. The hypothesis that the male sex has the least canalised sexual differentiation is supported by the observation that some old males developed oocytes.