Thiamineless dwarf colony variants of Staphylococcus aureus (D strains) were compared with their homologous normal colony variants and with normal S aureus. The majority of D strains tested in vitro exhibited biological features typical of S aureus. D strains, however, produced lower titers of α hemolysin and this toxin was also produced by a smaller proportion of these strains. D strains and S aureus strains were equally susceptible to disinfectants used in dairy farming and were isolated with similar frequency from the skin of the udders of infected cows. Attempts at inducing and selecting D strains from normal S aureus by exposing the latter to antibiotics in vitro and in vivo have so far failed.