Proventriculitis and chronic respiratory disease were diagnosed in two flocks of gray partridges (Perdix perdix) on unrelated Swedish game bird farms. Affected birds showed loss of condition, respiratory signs, and flock mortality rates of 50 and 98%, respectively. The proventricular lesions were associated closely with fungal organisms that were microscopically indistinguishable from the ascomycetous yeast Macrorhabdus ornithogaster (former provisional name “megabacterium”). At necropsy, the proventriculi were swollen and hyperemic, and viscous mucus adhered to the mucosa. Proventricular hemorrhages were commonly detected, and one bird had proventricular rupture and peritonitis. Microscopically, mild to severe subacute to chronic lymphoplasmacytic proventriculitis, microabscesses, necrosis, epithelial metaplasia, disrupted koilin, ulcers, and hemorrhages were observed. Transmission electron microscopy of the proventricular microorganisms revealed a membrane-bound nucleus, vacuoles, ribosomes, microtubules in parallel arrays, and a two-layered cell wall but no mitochondria. Scanning electron microscopy of the proventricular epithelium demonstrated masses of organisms with occasional constrictions in parallel arrangement. Many of the birds also suffered from concurrent respiratory bacterial infections and/or gastrointestinal candidiasis. The clinical course and gross and microscopic proventricular lesions were similar to those described in psittacine and passerine pet birds colonized by M. ornithogaster–like microorganisms but differed from published case reports and experimental infections of chickens in which the clinical signs and lesions have been considerably milder. The findings presented in this paper suggest that mycotic proventriculitis, presumably associated with M. ornithogaster, may be a serious but possibly opportunistic, although unusual, disease problem in gray partridges on game farms.