SUMMARYThe toxicity of fipronil (5-amino-1[2,6-dichloro- 4(trifluoromethyl)phenyl-4(trifluorormethyl) sulfinyl]-1H-pyrazola-3-carbonitrate), to adult female bees was least to the alkali bee, Nomia melanderi, (LD50 = 1.130 pg/bee), intermediate to the honey bee, Apis mellifera, (0.013 μg/bee) and greatest to the alfalfa leafcutter bee, Megachile rotundata, (0.004 pg/bee), both in topical tests and tests involving fipronil residues on alfalfa (Medicago sativa) foliage. Adding an adjuvant to fipronil sprays changed the toxicity of fipronil to bees in residue bioassay studies with varying results with adjuvant and species of bee. Fipronil at 100 ppm and 500 ppm in feeders containing a sucrose/honey syrup caused a significant reduction in honey bee visitation. Spraying fipronil at 0.014 or 0.028 kg a.i./ha on flowering canola (Brassica napus cv. Legend) did not cause significant mortality of adult honey bees or reduce bee visitation.