The sweetpotato whitefly,Bemisia tabaci(Gennadius), is a major agricultural pest, causing over 200 million damage to cotton alone in the United States in 1992. To understand sweetpotato whitefly feeding behavior, we examined the effects of phosphorus nutrition and leaf age on sweetpotato whitefly host selection. Cotton (Gossypium hirsutumL.) seedlings grown either hydroponically in growth chambers or in sand culture in the greenhouse were exposed to sweetpotato whitefly adults when two fully expanded true leaves were present. Phosphorus deficiency reduced oviposition on true leaves by 40% in the growth chamber, and by 38% on the youngest expanded leaf in the greenhouse, but had no effect on oviposition on cotyledons in either environment. When individual leaves from the greenhouse were analyzed, host acceptance was significantly correlated with low leaf sucrose concentration, but appeared to be unrelated to amino acid concentration or amino acid:sucrose ratio. This suggests that host selection was based on minimizing osmotic stress to the insect rather than on maximizing amino acid ingestion.