Two distinctly different and stable forms of Staphylococcus aureus were isolated from the blood of each of two patients with staphylococcal sepsis. In each case, one form was hemolytic and the other nonhemolytic, although both had the same biochemical reactions, phage types, and antibiotic susceptibilities, and both were virulent for mice. Variant forms of S. aureus may be selected in vivo by host factors and may be responsible for causing and/or perpetuating infection.