Effect of basil leachate on the emergence of 10 weed species, beggarweed (Desmodium tortuosum), crabgrass (Digitaria ciliaris), guineagrass (Panicum maximum), Johnsongrass (Sorghum halepense), milkweed strangler vine (Morrenia odorata), pigweed (Amaranthus retroflexus), sicklepod (Senna obtusifolia), Spanish needles (Bidens pilosa), teaweed or prickly sida (Sida spinosa) and yellow nutsedge (Cyperus esculentus) alongwith basil (Ocimum sanctum) was studied in Petridish and under pot conditions. In another study, intra- and interspecific competition between basil and 10 weed species (guineagrass) substituted with crowfoot grass (Dactyloctenium aegyptium) was carried out in pots under greenhouse conditions in a replacement series with four plants per pot. Effect of weed : basil mixture on fresh weight and plant height was observed. Relative crowding coefficients (RCC), relative yield total (RYT) and agressivity index (AI) was calculated. Basil leachate collected from soil influenced weed emergence (both inhibitory and stimulatory), but the differences between leachate or water used for irrigation in pots and Petri dish studies were not large enough except on some species, indicating any significant role of allelochemicals. Basil leachate inhibited the emergence of basil itself. Similarly, soil incorporation of basil root/shoot matter in 1 : 12 ratio (basil : soil) had no inhibitory effect on the emergence of D. tortuosum, S. halepense, M. odorata, A. retroflexus, S. obtusifolia, B. pilosa and C. esculentus. Basil suppressed the growth of all weed species, except yellow nutsedge, but plant height of basil was unaffected by weeds (when data averaged over weed species and plant ratio). Plant height of basil increased when growing with weed species compared to monoculture due to inter-specific competition. Even single plant of basil (with three of weeds) was competitive enough to reduce the plant height of D. aegyptium, S. spinosa, A. retroflexus and M. odorata; plant height increased significantly when weeds were growing in monoculture. RYT values of 1.04 in 1 : 3 mixture of basil with weeds compared to 0.83 in 3 : 1 or 2 : 2 ratio indicated some degree of resource complimentarity. Highest RYT value of 1.64 was derived for D. aegyptium followed by D. tortuosum (1.22), S. spinosa (1.14) and C. esculentus (1.06) when growing in 1 : 3 ratio with basil. RCC (basil on weeds and weeds on basil) values were significantly different for C. esculentus compared to other weed species when growing in 2 : 2 ratio. The aggressivity index of basil significantly decreased when one plant of basil was growing with three of weed species compared to 2 : 2 or 3 : 1 ratio (basil : weeds); lowest being with C. esculentus. The results suggest that insignificant effect on weed seed emergence with basil leachate may not be due to any allelopathic effect as the growth inhibition (plant height and fresh weight) does not support this hypothesis, but suggests dominant role of competition when weeds and basil were growing in different plant ratio.