Abstract
Selection of oviposition sites on two different types of media was studied for Alexandria (Egypt) and Florida populations of Drosophila melanogaster and D. simulans in population cages at 25 C. Differences between the two species in their oviposition on the two media sites and food types are discussed in terms of natural selection. Several interactions between the two species crud their experimental environments were demonstrated. It was argued that the interactions probably arose through natural selection which has promoted the coexistence of the two sibling species in the same environment. Differences in oviposition between the two geographical populations of both species are discussed.