After recovery, house finches (Haemorhous mexicanus) reinfected with the same Mycoplasma gallisepticum strain remain partially resistant to reinfection for at least 14 mo in that they recover from reinfection much more rapidly than do Mycoplasma gallisepticum-naïve birds. To test the response of birds to reinfection with a heterologous strain we performed two experiments. In a first experiment we exposed birds to one of three strains that differed in virulence. After they had recovered all were reinfected with the most virulent-strain available at the time of the experiment. In a second experiment we infected and later reinfected house finches with one of two Mycoplasma gallisepticum strains whereby we switched the order of the strain used. In both experiments, disease in birds reinfected with a more-virulent strain caused more-severe disease. Our data suggest that the observed increase in Mycoplasma gallisepticum virulence, once the disease has become endemic in free-ranging house finches is—in part—driven by increased resistance of recovered birds to strains of equal or lower virulence.