The brood sex ratio of the apple snail Pomacea canaliculata is highly variable among parents in areas where it has been introduced. Such variation in the sex ratio may reflect variation in the source population, or it may have stemmed from the mixture of multiple source populations with different sex-determining mechanisms. To distinguish these hypotheses, the sex ratios of F₁ (hatchlings from wild-collected egg masses) and F₂ snails (produced from crosses of the F₁ snails) from three sites in the native range in Argentina were studied. The sex ratios of the F₁ snails were not significantly different from 0.5, although there was a slight but significant difference among populations. The F₂ broods from intra-population crosses also lacked variation in the sex ratio. However, the sex ratios in F₂ broods from inter-population crosses were highly variable, from 0 (all females) to 0.84 (mostly males). Thus, the large variation in the sex ratio in areas where P. canaliculata has been introduced is probably due to the mixture of source populations.