The importance of the cotton aphid, Aphis gossypii Glover, as a mid-season pest in California cotton has increased since the early 1990's. Field experiments were conducted to further study aspects of cotton aphid biology and the effects of cultural control measures at the individual and population level. Previous laboratory and field studies have shown that high rates of nitrogen fertilizer can promote the build-up of aphid infestations. The goal of the present study was to determine the mechanisms behind this positive response of aphids to nitrogen. Aphid populations in 2000, at least in our study site, were characterized by overall moderate to high levels which developed during the mid-season. Naturally occurring aphid populations were monitored in 40 small plots with 10 differential fertilizer regimes. These fertilizer treatments included six nitrogen levels (0[=20 lbs./A soil residual nitrogen], 50, 100, 150, 200, and 250 lbs./A of ammonium sulfate), and four different "balanced" fertilizations (i.e.,different levels of nitrogen + K ₂₀ fertilizers). There was a positive trend for more aphids in the 200 and 250 lbs./A nitrogen treatments compared with the 0 and 50 lbs./A treatment with a 5-7X range across the treatments. Aphid densities on the highly fertilized plots exceeded the mid-season treatment threshold. Overall, the "balanced" fertilization treatments had a negative impact on the aphid populations but aphid densities still reached the treatment threshold. Detailed studies on cotton aphid fitness showed that aphid generation times, from a laboratory colony out-planted into field cages, ranged from 7.9 to 7.1 days and the number of offspring per adult averaged from 18.5 and 44.1 under 0 and 250 lbs./A nitrogen regimes, respectively. The potassium treatments had a moderate negative effect on both the generation time and the fecundity of the aphid. Similar patterns were observed when we repeated the experiments with a second and third aphid generations. These changes in aphid fitness at the individual level may explain in part the effects of nitrogen fertility at the population level observed in the field