It is still a suspicion among some evolutionary biologists that the incursion of molecular biology into their field will do little more than determine the DNA sequence differences underlying evolutionary changes already evident at the organismal level. Work on an esterase enzyme involved in the reproductive biology of Drosophila belies this view. Although it is already one of the most intensively studied gene - enzyme systems at an organismal level, recent molecular invetigations reveal several unexpected, and, in some cases, still inexplicable phenomena in its evolutionary history.