The effect of cotton aphids, Aphis gossypii (Glover), on cotton plant development and yield was studied during 2000 and 2001. Tests were conducted at the Macon Ridge Station (Winnsboro, Louisiana) in 2000 and 2001, and at the Northeast Research Station (St. Joseph, Louisiana) in 2001. Tests were performed in irrigated and non-irrigated regimes using individual plant comparisons, micro-plots, and whole plots. In both years and at both locations, cotton aphid densities induced chlorosis, and a downward cupping of leaves during the infestation period. The insecticides, pymetrozine and thiamethoxam, were applied to control natural infestations of cotton aphids. Although pymetrozine and thiamethoxam significantly reduced cotton aphid densities (P < 0.05), the aphicides did not provide 100 percent control. In both years, the entomopathogenic fungus, Neozygites fresenii, established epizootics in late June or early July and rapidly reduced cotton aphid densities to near zero. Cotton aphids did not influence plant height, vegetative branch number, sympodial fruiting node number, vegetative boll number, sympodial bolls, sympodial fruiting positions, and plant maturity measured at the time of harvest. Also, cotton aphid feeding did not influence seedcotton yields from whole plots, micro-plots, and individual paired plants. No significant differences were observed between aphicide treated and non-treated plots for percent retention of first, second, and third fruiting position bolls, and seedcotton weights from first, second, and third fruiting position bolls. Cotton aphids caused transient injury symptoms during the period of infestation, but did not influence final crop maturity, total seedcotton weights, or yield distribution on plants.