Under 12 hour light : 12 hour dark conditions, both per^VT1.1 and per^VT1.2 flies show the normal trend towards longer mid-day siesta and later evening activity as daily ambient temperature increases. However, in per^VT1.1 flies, the onset of evening activity is advanced and the mid-day siesta is shorter compared to per^VT1.2 flies. Differences in mid-day siesta and the timing of evening activity are most readily observed at 18[o]C. No significant differences in free-running behavioural rhythms have been observed between per^VT1.1 and per^VT1.2 flies at either 18 or 29[o]C.