Cultivation of resistant or tolerant varieties is the easiest way to protect the mustard crop from insect pests. Varietal screening for aphid resistance and stability of seed yield under aphid-infested and protected environment would help in identifying the tolerant varieties for aphid attack. The present investigations were therefore carried out with the objectives to study the evaluation of 240 B. juncea accessions for resistance/tolerance against mustard aphid. Under field conditions, the experiment was conducted with 240 accessions (germplasm) of Brassica junceaat Norman E. Borlaug Crop Research Centre (CRC), Pantnagar, during 2010-2011 to evaluate their differential response on the infestation of Lipaphis erysimi (Kaltenbach). Accessions 491128 491176, 491123, 491144 were highly resistant, and 491041, 320648,426401, 491042 were highly susceptible against mustard aphid based on its population. On the basis of mustard aphid infestation index (AII) the mustard germplasm, 339589 (0.46), 33957, 399816 were found highly resistant, whereas 296703, 491028, 248993, 296690 were highly susceptible