Morphological changes induced by a newly described avian virus in budgerigar (Melopsittacus undulatus) tissues were examined with light and electron microscopes. Infected cells viewed with the light microscope were found to have enlarged nuclei containing marginated chromatin. Cytoplasmic contents were frequently clear in appearance. Tissues affected included skin, feather, follicle, kidney, uropygial gland, crop, lung, liver, heart, bone marrow, spleen and brain. Serum from infected birds contained viral particles. No tumours were found in affected birds. Electron microscopy demonstrated viral particles that were naked, predominantly icosehedral, and 42-49 nm in diameter. Occasional elongate forms of the virions were seen in kidney tissue. Small groups of virions were occasionally enclosed within nuclear and cytoplasmic membranes. Viral particles were observed in chicken embryo fibroblast cultures 18 hours post-inoculation. The particles first appeared in swollen nuclei and subsequently were found in the cytoplasm of more senescent cells. Cytoplasmic disruption and swollen rough endoplasmic reticulum were also observed in infected cells. Negatively stained preparations of the fluid from infected chicken embryo fibroblast cultures contained typical cubic viral particles as well as elongate forms similar to those seen in excised tissue preparations.