From 1994 to 1999 in the Lower Fraser Valley region of southwest Canada, fonofos (Dyfonate G) was recommended for control of introduced wireworm (Agriotes spp.) pests on potato and other root crops. As part of a wildlife-monitoring program, we collected 15 raptors, including 12 bald eagles (Haliaeetus leucocephalus), found dead or debilitated on or near agricultural lands with severely inhibited brain and/or plasma cholinesterase activity and fonofos residues in ingesta. Bird remains, in nine cases waterfowl, were identified in the ingesta samples. Another seven bald eagles had severe cholinesterase inhibition, but without evidence of fonofos residues. During two winters from 1996 to 1998, 420 ha of potato fields, half of which had been treated the previous spring with fonofos and the remainder untreated, were searched weekly for evidence of wildlife mortality. Search efficiency was assessed with placed duck carcasses. Waterfowl outnumbered other species in field-use counts and comprised the greatest proportion of birds found dead. We found 211 wildlife remains, most scavenged; 35 intact carcasses were suitable for postmortem examination and/or toxicology analyses. Cholinesterase activity was assayed in brains of 18 waterfowl, five of which had severely depressed activity (average inhibition 74%; range, 69-78%). The gastrointestinal tract of a mallard found in a field treated with granular product contained 49 microg/g fonofos residues, linking waterfowl mortality with labelled use of the product. These findings demonstrate the risk of both primary and secondary poisoning by anticholinesterase insecticides where wildlife make intensive use of farmed fields.