Several commercial broiler flocks in northeastern Georgia that were the progeny of the same parent flock (Flock 40) were diagnosed as Mycoplasma gallisepticum (MG) positive by serology, culture, and PCR. Flock 40 had been vaccinated with ts-11 live MG vaccine. Several isolates were obtained from the MG-positive broiler flocks, and these isolates were indistinguishable from the ts-11 vaccine strain by the molecular strain differentiation methods used. A pathogenicity study was performed to compare the virulence of one of the isolates, K6216D, to the ts-11 vaccine strain. K6216D elicited a significantly stronger antibody response and significantly increased colonization of the tracheas and air sacs. K6216D also elicited significantly greater air sac and tracheal lesions than the ts-11 vaccine strain at 10 and 21 days postinoculation (P < or = 0.05). This is the first report of a field case of the apparent reversion to virulence and vertical transmission of the ts-11 vaccine.