Group A streptococcal pyrogenic exotoxins (SPEs A, B, and C) are superantigens. SPE A shares high sequence similarity withStaphylococcus aureusenterotoxins (SEs) B and C. Since SPE A is bacteriophage-encoded, we hypothesized that its gene (speA) was acquired fromS. aureus.speA, when cloned intoS. aureus, was stably expressed, its protein resistant to proteases, and the gene under accessory gene regulator control.speAwas acquired by streptococci from cross-species transduction.speBwas not expressed inS. aureus.SPE C was degraded by staphylococcal proteases. The genesspeBandspeCwere not recently acquired fromS. aureus.