Sixteen pregnant female common shrews were collected near Oxford (U.K.) from a hybrid zone between two karyotypic races which differ by Robertsonian rearrangements. Some females were homozygotes and others were ‘simple’ or ‘complex’ Robertsonian heterozygotes. The females and their fetuses were karyotyped and number of ovulations and regressing implants scored. Prenatal losses were noted in female simple Robertsonian heterozygotes. Some losses, including two trisomics, were likely to be due to anaphase I nondisjunction of the maternal autosomal trivalent. However, numbers of ovulations were found to be higher in these heterozygotes than in the homozygotes, and this could compensate for prenatal losses due to nondisjunction. Overall, fertility of females from the hybrid zone did not differ substantially from that of females from other areas of karyotypic polymorphism or monomorphism. There was no evidence for assonative mating or assortative fertilisation within the hybrid zone. On chromosomal grounds, the hybrid zone is unlikely to be a major barrier to gene flow.