Twelve helmeted honeyeaters (Lichenostomus melanops cassidix), 11 yellow-tufted honeyeaters (Lichenostomus melanops gippslandica), 3 purple-crowned lorikeets (Glossopsitta porphyrocephala), and 3 little lorikeets (Glossopsitta pusilla) died over a short period of time with acute thiamine deficiency. Diagnosis was based on polioencephalomalacia observed histologically, low brain thiamine levels (<1 mg/kg), and the absence of other significant findings on tests performed. The birds were maintained on an artificial nectar (water, honey, and a multivitamin supplement) for 2 years, but this diet was analyzed to be low in thiamine (0.001 mg thiamine/ml of diet). An additional secondary thiamine antagonist, not identified, may have precipitated the mass mortality.