The circadian clock that regulates behavior and physiology of animals is synchronized to the natural, 24 hour, light-dark cycle primarily by light. In Drosophila melanogaster, light is transduced to the clock neurons via the external photoreceptors in addition to the blue-light photopigment, CRYPTOCHROME (CRY), which plays a special role in clock phase resetting. Previous studies showed that the y w mutation in D. melanogaster is correlated with altered locomotor and molecular circadian rhythms, probably due to their disturbed eye pigmentation and sensitivity, and neurotransmitter physiology. This study investigates the relationship between this mutation and the CRY expression profile as a primary player in the photic entrainment pathway. Results revealed that CRY levels was most of the time higher in y w mutants compared to the wild-type CS fruit flies in all clock neuronal clusters expressing CRY. The reduced and delayed degradation of CRY indicates week photoreception by the mutants and provides a possible explanation for the delayed PERIOD (PER) and suppressed Pigment-Dispersing Factor (PDF) signaling detected previously. It could also explain the delayed morning and advanced evening locomotor activity peaks of these y w mutants. The results also showed a higher level of variation in CRY staining between individual neurons within each cluster, probably due to reduced PDF signaling which is known to internally synchronize clock neurons. It could be concluded that, the mutations affecting the photic entrainment pathway to the circadian clock, like the y w mutation, would probably have widespread effects on the circadian clock machinery and the overt rhythms under its regulation