We investigated the effects of a turf application of the insecticide diazinon AG500 on Canada geese (Branta canadensis) on a golf course in coastal Washington (USA). On both 19 and 26 March 1987, 1 ha of turf on a golf course located in Birch Bay, Washington was treated with diazinon AG500 at a target application rate of 2.2 kg active ingredient per hectare (Al/ha). Treated areas were then irrigated with 6 mm water. Grass and water samples were collected from three different sites one day before and 1, 3, 7 and 14 days after each application. Diazinon residues ≥20 ppm were found in golf course grasses for one week after each application. Diazinon residues in study area ponds and creeks were ≥17 ppb. Samples from two irrigation puddles one day post-application had 1.00 and 0.20 ppm of diazinon, respectively. Numbers of geese present declined following diazinon application; however, no goose mortality was observed. Geese spent 422 and 538 min feeding on the treated areas after the first and second diazinon applications, respectively. One goose feeding in treated areas demonstrated signs of poisoning (lethargy, ataxia) for several hours. Two other geese feeding in the treated areas may have been slightly intoxicated. During carcass searches, three American wigeon (Anas americana) carcasses were found. Based on brain cholinesterase (ChE) levels and gastrointestinal (GI) tract residues of diazinon present, we concluded that these wigeon died from diazinon poisoning. Numerous songbirds (Passeriformes) also fed on the treated turf but no apparent response to the insecticide was observed.