Differences between strains set up from single inseminated founder females collected in the wild were found in both Drosophila melanogaster and D. simulans, for the number of interspecific crosses giving offspring. The differences were mainly controlled by the strain of the female, but to a lesser extent by the strain of the male, especially in D. simulans. More crosses with offspring occurred over-all for female D. melanogaster strains, as compared with female D. simulans strains. The variability between strains implies that the founder females collected in the wild differ. Thus, in natural populations, there is variability determining the likelihood of offspring in interspecific crosses determined mainly by the strain of the females in the intraspecific populations.