DES 119 cotton (normal, hairy leaf) and its smooth-leaved isoline were monitored for cotton aphid, Aphis gossypii Glover, densities and effects on yield in aphicide-treated and untreated plots. The interaction of cotton isoline and aphicide treatment regime was significant for aphid density and cotton yields. Aphids caused a significant yield loss (about 125 lbs. lint/acre) in the normal, hairy isoline of DES 119 cotton when it was not treated with aphicide. No yield differences were observed between aphicide-treated and untreated plots containing the smooth isoline of DES 119. Yields for the hairy isoline with aphicide, the smooth isoline without aphicide, the smooth isoline with aphicide, and the hairy isoline without aphicide were 985.9, 888.9, 866.3, and 860.0 lbs. lint/acre, respectively. The hairy isoline treated with aphicide yielded significantly greater than the other three treatments. No differences were observed in the densities of, nor damages caused by, major cotton pests which could account for the observed yield loss.