(Z)-9-tricosene (muscalure), a material produced by female house flies, Musca domestica L., previously has been shown to attract male house flies in olfactometer tests. It was here shown also to induce the sex-behavior pattern of male house flies in tests with fly models (the pseudofly-petri dish test), whereas (E)-9-tricosene, which induced little if any activity in the olfactometer tests, was similarly inactive in the pseudofly-petri dish tests. Muscalure appears to account for the phenomena measured in both test systems.