Influences of nitrogen fertilization on grain yield and nitrogen fixation of cowpea [Vigna unguiculata (L.) Walp. cv. ‘California Blackeye No. 5’] were studied under well-watered and dry field conditions. The soil was an Arlington fine sandy loam (coarse-loamy, mixed thermic Haplic Durixeralf). Nitrogen fertilization treatments involved applications to the soil either at sowing or early flowering, and foliar applications during pod filling, of 60 kg N/ha. Similar relative yield responses to fertilizer treatments were observed with cowpea subjected to both well-watered conditions and late drought. Nitrogen application at sowing had little effect on grain yield or N fixation from 45 days to the end of the season, whereas later applications to the soil or foliage substantially increased grain yield (by 300–600 kg/ha). Early-flowering soil application increased the numbers of branches and pods/m² and reduced N fixation, whereas foliar fertilization delayed leaf senescence and increased seed weight. Applying N fertilizer both to the soil at early flowering (60 kg/ha) and to foliage (60 kg/ha) reduced N fixation, and increased both number of pods/m² and seed weight. However, grain yield was increased only to the same extent as when fertilizer was applied to either the soil or the foliage alone. Folian, which contained N, P, K, S, and Fe, was a more effective foliar fertilizer than urea alone, although N was probably the major nutrient responsible for the increases in grain yield. Higher concentrations of Folian (33 g N/L) caused leaf burn, and did not increase yield as much as lower concentrations (22 g N/L and 11 g N/L), which did not cause leaf burn and increased yield significantly. Drought reduced both N fixation and the extent to which plants depended on fixed N for their total N supplies.