We investigated effects of apterous mutation ap56f on circadian locomotor activity, eclosion rhythms, and transcript levels of period and timeless in Drosophila. We investigated circadian locomotor activity and eclosion rhythms in ap56fand wild-type flies, their F1 and F2 offspring, and wingless vestigial mutants and show that ap56f disrupts circadian locomotor rhythms in a genetically recessive manner, that is not caused by the absence of wings. The apblt strain also showed impaired circadian activity rhythms, providing independent evidence for a significant role of apterous in circadian locomotor rhythm expression. The ap56f mutation did not disrupt a circadian eclosion rhythm or rhythmic expression of the period and timeless clock genes, indicating that apterous is not essential for circadian clock function, but is necessary for coupling locomotor activity to a circadian clock. Timeless transcription was reduced in ap56f flies in 12:12 LD, suggesting that apterous may modulate core clock gene expression.