Abstract
Interspecific crossability between two sibling species, Drosophila melanogaster and D. simulans, were examined in nature and in the laboratory. About 4.6% of D. melanogaster females were found to be inseminated by D. simulans males in six Japanese natural populations. In the laboratory experiments D. melanogaster females collected and established from non-sympatric or newly sympatric sites with D. simulans were more easily inseminated than those from historically sympatric sites. It may be suggested that the premating isolation does not develop rapidly after the sibling species come sympatry.
