“Toxicological studies of imidacloprid and bifenthrin against mustard aphid,Lipaphis erisimi (Kalt.) on Brassica rapa L. subsp. Oleifera (toria)” revealed that all the incecticidal treatments viz., imidacloprid 20, 40, 60 g a.i./ha, bifenthrin 60, 80, 100 g a.i./ha and deltamethrin 15 g a.i./ha were found to be significantly superior over control (untreated). The highest population reduction of the aphid was obtained with imidacloprid at 60 g a.i./ha (81.59 and 86.01%) after one and seven days of spraying in both the years. The highest mean yield was obtained from imidacloprid 60 g a.i./ha (6.97 q/ha)which was not significantly different from imidacloprid 20 g a.i./ha . The LC50 values of imidacloprid was 0.00017.Based on population reduction, yield,cost : benefit ratio, relative toxicity, residual toxicity and residues dissipation pattern imidacloprid 20 g a.i./ha can be considered sufficient to control the L. erysimipopulation and to optimize the yield of rapeseed.