Field tests were conducted to determine the influence of cotton aphid, Aphis gossypii Glover, infestations during the early (vegetative) and mid-season (flowering) periods on cotton yields. Treatments were arranged in a split-plot within a randomized complete block design. Main blocks consisted of irrigation regime (irrigated vs. non-irrigated) and subplots consisted of aphicide treatment (aphid control vs. no control). Aphicides included were pymetrozine (Fulfill 80WP, 0.086 lb AI/acre) in 2000 and thiamethoxam (Centric 25WG, 0.047 lb AI/acre) in 2001. Plants were sampled weekly to determine cotton aphid density and to monitor the occurrence of the entomopathogen, Neozygites fresenii. Seedcotton was harvested from sub plots and from one meter micro-plots within each sub plot. Plants within each micro-plot were tagged at the corresponding mainstem node that N. fresenii eliminated cotton aphid populations. Seedcotton yields within micro-plots also were partitioned into cotton aphid infested zones, non-infested zones, and vegetative branches on plants.