Naturally-occurring mycoplasmal conjunctivitis is described among 104 wild-caught, and initially seronegative, house finches (Carpodacus mexicanus) maintained in captivity for 12 wk during November 1995 through January 1996. Finches housed in three pens were monitored for clinical signs, and > or = 10 birds were euthanatized for necropsy and mycoplasma testing every 2 wk. Within 2 to 4 wk following initial detection of lesions, > 50% of the birds in each of three pens developed a debilitating disease characterized by mild to severe ocular swelling, conjunctivitis, and ocular and nasal discharge. Microscopic lesions in affected finches consisted of mild to severe lymphoplasmacytic inflammation with epithelial and lymphoid hyperplasia in conjunctivae, nasal turbinates, and trachea. Mycoplasma gallisepticum infection was confirmed by culture or polymerase chain reaction (PCR) in all birds with conjunctival lesions and in 43% of birds without lesions. An arbitrary primer PCR was used to confirm M. gallisepticum isolates as identical to a field strain previously associated with house finch conjunctivitis. Most birds (89%) with conjunctivitis developed a concurrent antibody response detectable by serum plate agglutination (SPA) within 2 wk of lesion development. Hemagglutination inhibition (HI) and enzyme-linked immunosorbent assay (ELISA) tests were less sensitive than the SPA test. The clinical severity of this disease and high proportion of affected birds suggests that M. gallisepticum may have a negative impact on free-flying house finch populations.