During the second half of the year 2000, we have diagnosed a megabacterial-associated proventriculitis in 13 laying hens and 4 cocks from 14 different flocks and one turkey. All birds were submitted for necropsy because of progressive runting, increased mortality and poor laying performance in the herds. Routine diagnostic workup included necropsy, histology, bacteriology, parasitology and virology. At necropsy, the proventricular of the birds were enlarged. The walls were thickened and the mucosa covered with cloudy, grey-white mucus. Petechial haemorrhages and ulcerations predominantly at the proventricular-ventricular junction and sloughing of the necrotic koilin layer of the ventriculus were inconsistently present. Megabacteria (approximately 40-50 by 2-4 microns, gram-positive to gram-labile, rod-shaped organisms) were present in large numbers in touch preparations of the proventricular mucosal surface. In some of the preparations, the megabacteria showed a branching pattern similar to fungal hyphae. By histopathology, all birds showed moderate to marked, diffuse lympho-plasmacytic proventriculitis. Heterophilic exudation was present in the terminal portion of the proventriculus, where the greatest numbers of the organism were found. Megabacteria were present in the mucus covering the mucosal surface and the lumina of the superficial proventricular crypts, occasionally invading the luminal epithelium. Megabacterial infection was in general associated with other diseases such as avian tuberculosis, salmonellosis, coccidiosis, chlamydiosis and various other bacterial and parasitic infections. Only in one hen no other concurrent infectious organism was detected. The turkey had histomoniasis. Wild birds were regarded as probable source of infection, since all affected birds were kept under conditions allowing contact to wild birds and we have diagnosed megabacterial infections in wild-living green finches earlier.