Mycoplasma gallisepticum conjunctivitis emerged in 1994 as a disease of free-ranging House Finches (Carpodacus mexicanus) in North America and has also been isolated from other songbirds with conjunctivitis. A key feature for the successful study of natural and experimental disease has been the apparent, very-high correlation between characteristic eye lesions and M. gallisepticum. Mycoplasma sturni was originally isolated from an adult European Starling (Sturnus vulgaris) with bilateral conjunctivitis and has since been reported in a relatively small number of other avian species, but not in House Finches. We identified as M. sturni a mycoplasma isolate from a California House Finch with conjunctivitis. However, experimental infection of House Finches with the M. sturni isolate failed to reproduce the disease. Therefore, M. gallisepticum remains the primary known cause of conjunctivitis in House Finches.