There exists in Drosophila a class of mutations called “homeotic” that cause changes in determination of the imaginal discs. The homeotic mutant, ss^a40a, was found to have a temperature-sensitive transformation of the arista segment of the antennal complex to a tarsus of the leg. In a selected stock penetrance was complete, so that at 29°C normal aristae were produced, whereas at 17°C complete tarsi developed in all flies. “Temperature-shift” studies revealed a temperature-sensitive period in the third larval instar. The temperature-initiated action of ss^a40a does not appear to result from a simple wave or gradient of determination passing through the disc. Shifts up during the temperature-sensitive period yielded leg tissue proximally, which increased distally with progressively later shifts. Shifts down yielded leg tissue at the aristal base, which retreated with later shifts.