Mollicutes (mycoplasmas) are pathogenic in a wide range of mammals (including humans), reptiles, fish, arthropods, and plants. Of the medically important mollicutes, Mycoplasma gallisepticum is of particular relevance to avian agriculture and veterinary science, causing chronic respiratory disease in poultry and turkey. Using two-dimensional electrophoresis based quantitative expression proteomics, the current study investigated the molecular mechanisms behind the phenotypic variability between a M. gallisepticum vaccine strain (6/85) and a competitive, virulent field strain (K5234), two strains which were indistinguishable using commonly accepted genetic methods of identification. Twenty-nine proteins showed a significant variation in abundance (fold change > 1.5, p-value < 0.01). Among others, the levels of putative virulence determinants were increased in the virulent K5234, while the levels of several proteins involved with pyruvate metabolism were decreased. It is hoped that the data generated will further the understanding of M. gallisepticum virulence determinants and mechanisms of infection, and that this may contribute to the optimization of diagnostic methodologies and control strategies.