Two episodes of accidental urea toxicosis are described in wild silver gulls (Larus novaehollandiae) following spillage of fertilizer grade urea at a commercial shipping facility near Perth, Western Australia. In both cases, urea spillage had been seen to contaminate freshwater wash-down pools on the wharves where ships were being unloaded and gulls were seen to be drinking and washing in the pools nearby the spillages. Affected birds were found moribund or dead. Necropsy and histopathological findings were non-specific and consisted of mild to moderate congestion of visceral organs and brain. Analysis of a water sample collected during Case 1 revealed a very high urea concentration of 4.124 mol/l (pH 5.5), and fluid from the proventriculus of two birds had urea concentrations of 382 and 308 mmol/l, respectively. Nine birds were examined during the second episode (Case 2) and, from heparinized heart blood samples collected (n = 5), the mean plasma urea (288 ± 92.0 mmol/l), ammonia (43.9 ± 34.2 mmol/l) and uric acid (7.45 ± 1.99 mmol/l) concentrations were markedly elevated above the reference ranges for all bird species. Proventricular contents (n = 7) similarly contained high concentrations of urea (394 ± 203 mmol/l) and ammonia (9.3 ± 15 mmol/l). The probable mechanisms of urea and ammonia toxicity in these birds are discussed.