Fipronil was applied to cotton by ground sprayer at ultralow volume and conventional spray rates. Boll weevil mortality and residue analysis of fipronil and its metabolites at 0,1, 2, 3, and 6 d after application were used to evaluate treatments. An evaluation of the toxicity of fipronil and metabolites to boll weevils also was conducted. The order of toxicity (LD 50 ) of fipronil metabolites at 48 h was sulfide > fipronil > sulfone > desulfinyl. Differences in percentage of mortality between treatments did not occur until 3 d after application when the ultralow volume application had higher mortality (98%) than the conventional (85%) spray rate. There were no differences in mortality between treatments at 6 d after application. Fipronil residues were present on the leaf surface longer and in higher amounts when applied in cottonseed oil at 1.17 liter/ha than when applied in water as a conventional application at 97 liter/ha.