Homologs of the Drosophila clock gene per have recently been cloned in Lepidopteran and Blattarian insect species. To assess the extent to which clock mechanisms are conserved among phylogenetically distant species, we determined whether PER protein from the silkmoth Antheraea pernyi can function in the Drosophila circadian timing system. When expressed in transgenic Drosophila, the silkmoth PER protein is detected in the expected neural cell types, with diurnal changes in abundance that are similar to those observed in wild-type fruitflies. Behavioral analysis demonstrates that the silkmoth protein can serve as a molecular element of the Drosophila clock system; expression of the protein shortens circadian period in a dose-dependent manner and restores pacemaker functions to arrhythmic per0 mutants. This comparative study also suggests that the involvement of PER in different aspects of circadian timing, such as period determination, strength of rhythmicity, and clock out-put, requires distinct molecular interactions.