A field experiment was conducted on medium  deep black soil at Agricultural Research Station, Dharwad during kharif season of 2005 and 2006 under rainfed conditions to study the effect  of black polyethylene mulching (75,100 and 125  micron thickness ) on growth, yield components and seed cotton yield of different cotton genotypes (RCH Bt, DHB-290, DHH-11, Sahana, Jayadhar and DLsa-17) . Pooled data of two years indicated that the seed cotton yield of no mulch was 1708 kg per ha and it significantly increased with 75 micron  (1989 kg/ha),100 micron(1894 kg/ha) and 125 micron (2042 kg/ha) polyethylene mulch. Among the genotypes tried, RCH-Bt hybrid recorded the highest seed cotton yield (2784 kg/ha).It was followed by DHH-11 (2158 kg/ha), Sahana variety (2032 kg/ha) and DHB-290 hybrid (1804 kg/ha). Significantly low yields were recorded with Jayadhar (1262 kg/ha) and DLSa-17 (1409 kg/ha).