Results are presented from a microbiological (bacteriological and mycological) investigation and the sensitivity tests of microorganisms isolated over a 2-year period (1983-84) from 80 fecal samples and 466 necropsies of psittacine birds. These results are correlated with signs of metaplasia of the salivary glands due to vitamin A deficiency in large parrots and with low vitamin A levels in the livers of small psittacine birds. Aerobic microorganisms were isolated from 76% of the fecal samples and 67% of the necropsies. Infections with primary pathogenic bacteria were found in less than 10% of the necropsies. Metaplasia was found in 51% of the large parrots, and the vitamin A levels in budgerigars were below acceptable levels. The possible role of a deficient diet in Psittaciformes in the occurrence of microbial infections and their treatment are discussed.