The rest/activity rhythm is an innate ubiquitous and complex biological phenomenon. It is essential/crucial for an organism's survival such as feeding, mate choice, escape from the predator, etc. Studies have shown that locomotor behaviour is sensitive to environment. The present investigations were made to understand the effect of different light regimes on locomotor behaviour of the two co-existing species of montium, a subgroup of Drosophila. Locomotor activity rhythms were assayed using Drosophila Activity Monitor. The activity of two species was analyzed in light/dark (LD), continuous dark (DD) and continuous light (LL) for 15th and 30th generations and the amount of activity was measured. The results revealed that the mean activity recorded for D. gangotrii and D. jambulina was significant. Unlike that of Drosophila melanogaster, these species entrain to LD, arrhythmic in LL and free runs in DD. Activity was either unimodal or bimodal. There was tremendous reduction in the activity in 30th generatio...