Topical bioassays of fipronil against a field-collected and laboratory reference strain of boll weevil from the Lower Rio Grande Valley showed equal LD50 values.The LD50's of field-collected strains from Louisiana and Lower Rio Grande Valley were equal. In Texas in 1993 (Test 1), 15 applications of fipronil applied at 0.028, 0.056, and 0.084 kg(a.i.)/ha were equally effective at reducing percentage damaged squares and bolls below the untreated check. In Texas in 1994 (Test 2), 14 applications of 0.056 kg(a.i.)/ha effectively reduced damaged fruit below that of the untreated check. Ultra low volume ground application tests in Mississippi in 1995 (Test 3) showed no differences in toxicity of two rates (1.02 and 1.36 kg(a.i.)/ha) of malathion and fipronil at 0.056 kg(a.i.)/ha. Fipronil applied ultra low volume by aircraft at rates of 0.043and0.056kg(a.i.)/ha(Test 4) were equally effective against boll weevils in bioassays of treated leaves. In a subsequent test (Test 5), 0.028 and 0.043 kg(a.i.)/ha rates of fipronil were equal in toxicity to boll weevils. Residues of fipronil + four metabolites on leaf surfaces determined during aerial tests were directly related to boll weevil mortality. Fipronil is highly effective against boll weevil in either low volume sprays in oil or high volume sprays in water.