Flies carrying Clk^{per.T:Ivir\HA1} have locomotor activity rhythms with normal period lengths of about 24 hours at 25^oC or 29^oC under constant darkness conditions. Most of the flies show robust rhythms. There are no differences in the waveform of the activity rhythm in Clk^{per.T:Ivir\HA1} and control flies under constant darkness conditions; the duration of morning activity, the portion of activity occurring during the subjective day and timing of the upswing, peak and downswing in either morning or evening bouts of activity are all normal in Clk^{per.T:Ivir\HA1} flies. The duration of locomotor activity following lights-on under 12 hour light:12 hour dark ( 12:12LD ) conditions is significantly longer in Clk^{per.T:Ivir\HA1} flies compared to control flies. The timing of the evening peak of activity starts about 30 minutes earlier than in control flies and Clk^{per.T:Ivir\HA1} flies show a significantly higher portion of their daily locomotor activity during the light phase. Brief light pulses given during the first day of constant darkness conditions elicit high levels of activity in both Clk^{per.T:Ivir\HA1} and control flies, but the bouts of activity last longer in Clk^{per.T:Ivir\HA1} flies.