A field experiment was conducted during rainy seasons of 2010-11 and 2011-12 at Guntur, Andhra Pradesh, India to evaluate the bio-efficacy of fluazifop-p-butyl against grassy weeds in cotton and its residual effect on succeeding crop of blackgram during the winter season. Results revealed that among different doses of fluazifop-p-butyl, post-emergence application of fluazifop-p-butyl 167 g/ha reduced grassy weed growth and recorded higher crop dry weight, yield components and seed cotton yield (1915 kg/ha) over its lower doses (100 and134 g/ha) but was on par with the highest dose of 335 g/ha in both the years. None of the doses of fluazifop-p-butyl including the highest dose (335 g/ha) did not cause any injury to cotton. In a field experiment, residual carry over effect of fluazifop-p-butyl at different doses of 100 to 335 g/ha was studied using blackgram as a succeeding crop. Differences were non-significant on plant population, plant height, crop dry weight and seed yield of blackgram indicating that there was no residual effect of fluazifop-p-butyl in the soil after harvesting of the cotton crop. It was concluded that the post- emergence application of fluazifop-p- butyl 167 g/ha was effective in controlling grassy weeds in cotton without any crop injury and residual effect on succeeding crop.