The circadian clock of Drosophila melanogaster consists of at least two interlocked feedback loops. In the first, the period and timeless gene products negatively regulate their own transcription. CRYPTOCHROME (CRY) is the dedicated circadian photoreceptor, and flies carrying a strong hypomorphic mutation in the cry gene have severely blunted circadian photoresponses. CRY physically interacts with the core components of the clock, PERIOD (PER) and TIMELESS (TIM) in a light-dependent manner. Previous work carried out in the laboratory showed that removing 20 amino acids at the C-terminus of CRY to create CRYDelta results in the loss of light-dependency of CRY interactions in yeast two-hybrid assays. Based on this work, the aim of my project was to study the role of the CRY C-terminus in vivo by clock neurons targeted overexpression of CRYDelta with the hypothesis that it should behave as a constitutively active form of the protein. CRYDelta flies have long period of locomotor activity in constant darkness, show abnormal responses to light and exhibit altered oscillation of the PER and TIM proteins in central and peripheral clocks. These phenotypes are reminiscent of responses observed when wild-type flies are kept under continuous low-light intensity. Therefore, this study provides strong behavioural, molecular and immunohistochemical evidence confirming that CRYDelta is constitutively active, and elicits continuous light responses. Moreover, previous work demonstrated that CRY role in the Drosophila clock exclusively involves light signalling to the core components of the clock. This study identified a potential new light-dependent function for CRY in vivo.