Depending on fitness consequences, hybridization may rescue inbred populations; generate premating barriers, reproductive interference, or hybrid species; or extinguish a species. However, the fitness of hybrids is unpredictable without direct quantification of their performance in fitness components across multiple generations. The land snails Bradybaena pellucida and B. similaris, which are indigenous and non-indigenous in Japan, respectively, copulate with each other simultaneously and reciprocally. However, only B. pellucida produces hybrids, because it ends mating by removing the penis before transferring a spermatophore, while B. similaris inseminates B. pellucida. To evaluate the strength of an intrinsic postzygotic barrier against the hybrids produced by B. pellucida, we conducted breeding experiments in the laboratory and measured six life-history traits: (1) growth rate, (2) body weight at maturity, (3) number of days to first oviposition after being permitted to mate, (4) clutch size, (5) fecundity, and (6) hatchability. We also calculated the relative intrinsic fitness based on five of these trait values (excluding clutch size). F(1) hybrids exhibited heterosis in growth rate, body weight at maturity and relative intrinsic fitness. F(2) hybrids also showed heterosis in body weight at maturity. Nevertheless, the F(2) hybrids produced significantly fewer progeny than the mid-point value of the parental species. Thus, the F(2) hybrids exhibited weak out-breeding depression in reproduction, offsetting their vigor in body size. These results indicate that only a weak postzygotic barrier, contrasting with strong F(1) heterosis, has evolved during genetic divergence of the two sibling species in allopatry. Article ZOOLOGICAL SCIENCE. 26(3):191-196 (2009)