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The use of controlled, horizontal-transmission experiments provides detailed information on the spread of disease within fixed social groups, which informs our understanding of disease dynamics both in an empirical and theoretical context. For that reason, we characterized in 2002, horizontal transmission of Mycoplasma gallisepticum (MG) in two flocks of 11 wild-caught house finches housed in outdoor aviaries over a 6-mo period. All birds were initially free of MG by a polymerase chain reaction (PCR)-based test, rapid plate agglutination (RPA), and the scoring of physical signs. We inoculated one flock member bilaterally in the palpebral conjunctiva and reintroduced it into its cage. Index birds developed conjunctivitis within 3 to 5 days but died 13 and 20 days postinfection (PI) possibly because of very severe weather. The proportion of birds with physical signs increased gradually, reached 40% at 6 wk PI, and fluctuated around 40% until 21 wk PI. By the time our experiment ended at 24.5 wk PI, 28% of the birds still exhibited physical signs. Across both flocks, 80% of the birds developed unilateral or bilateral conjunctivitis, and several birds relapsed. The appearance of physical signs in new individuals occurred between 10 and 144 days PI (median 41 days PI). Physical signs lasted 1-172 days (median 42 days). Birds that became infected earlier during the experiment developed more severe conjunctivitis, and there was a tendency for birds that developed bilateral conjunctivitis to develop physical signs earlier. Most birds that developed physical signs of MG were also PCR- and RPA-positive, although we detected a single asymptomatic carrier and a single symptomatic false negative. No birds died as a result of secondary MG infection.