The critical period of weed‐crop competition was studied in greengram(Vigna radiate)during the monsoon seasons of 1982 and 1983, the summer seasons of 1983 and 1984 and in blackgram(Vigna mungo)during the monsoon seasons of 1982 and 1983. The major weed species during the monsoon seasons wereEchinochloa colonum, Dactyloctenium aegyptium, Eleusine indica, Digitaria sanguinalis, Celosia argentea, Phyllanthus niruri, Cleome viscosa. Cyperus rotundus and C. iria.During the summer seasonsE. colonum, D. aegyptium, Physalis minima, Portulaca quadrifidaandC. rotunduswere the major weed species. Competition during the first 15 days after sowing (d.a.s.) in greengram during the summer season had only a small depressing effectongrain yields. Grain yields of both crops during two seasons increased significantly with the increase in weed‐free duration up to 30 d.a.s. However, a weed‐free condition maintained beyond 30 d.a.s. induced no further improvements in grain yields. Weed emergence and growth 30 d.a.s. were low and suppressed by the crops. The period during the first 30 d.a.s. for monsoon‐planted crops of greengram and blackgram, and 15–30 d.a.s. for a summer‐planted crop of greengram was found to be critical for crop‐weed competition.