The circadian pacemaker controlling the eclosion rhythm of the high altitude Himalayan strains of Drosophila ananassae captured at Badrinath (5123 m) required ambient temperature at 21 degrees C for the entrainment and free-running processes. At this temperature, their eclosion rhythms entrained to 12h light, 12h dark (LD 12:12) cycles and free-ran when transferred from constant light (LL) to constant darkness (DD) or upon transfer to constant temperature at 21 degrees C following entrainment to temperature cycles in DD. These strains, however, were arrhythmic at 13 or 17 degrees C under identical experimental conditions. Eclosion medians always occurred in the thermophase of temperature cycles whether they were imposed in LL or DD; or whether the thermophase coincided with the photophase or scotophase of the concurrent LD 12:12 cycles. The temperature dependent rhythmicity in the Himalayan strains of D. ananassae is a rare phenotypic plasticity that might have been acquired through natural selection by accentuating the coupling sensing mechanism of the pacemaker to temperature, while simultaneously suppressing the effects of light on the pacemaker.