Expression of cry^{Δ5'.Scer\UAS.cDa}, under the control of Scer\GAL4^tim.PE in flies exposed to constant darkness results in the flies exhibiting a significant increase in period length. The free-running period for these flies is 1.5-2.7 hours longer than for controls. The free-running period of flies expressing cry^{Δ5'.Scer\UAS.cDa}, under the control of Scer\GAL4^tim.PE is the same in cry⁺ and cry^b flies. Under constant high levels of light, flies expressing cry^{Δ5'.Scer\UAS.cDa}, under the control of Scer\GAL4^tim.PE remain synchronised for about 4 days (whereas control flies desynchronise and become arrhythmic after 1 or 2 days), but with a long period, suggesting that they are less sensitive to constant light. Flies expressing cry^{Δ5'.Scer\UAS.cDa}, under the control of Scer\GAL4^tim.PE phase-shift almost normally at ZT15 and ZT17, but at ZT19 remain in the delay zone, becoming nearly unresponsive at ZT21 and ZT23. Flies overexpressing cry^{Δ5'.Scer\UAS.cDa}, under the control of Scer\GAL4^tim.PE display a normal pattern of delay and advance but are more reactive to light pulses. cry^b flies expressing cry^{Δ5'.Scer\UAS.cDa}, under the control of Scer\GAL4^tim.PE generate a significantly enhanced phase shift response to a short light pulse, compared to cry^b and control flies at ZT15, but are essentially unresponsive at ZT21. cry^b flies expressing cry^{Δ5'.Scer\UAS.cDa}, under the control of Scer\GAL4^tim.PE, unlike cry^b mutants, do eventually desynchronise under constant light, and do phase-shift significantly in the delay zone of the phase-response curve.