Necrotizing fasciitis, which is a severe and uncommon infection involving the subcutaneous tissues, is usually caused by group A streptococci. To our knowledge, however, group B streptococci (Streptococcus agalactiae) have been reported to cause necrotizing fasciitis in only 4 instances (2 involving neonates) over the past 4 decades. We report 3 cases of group B streptococcal necrotizing fasciitis in adults that occurred in southern Ontario and Quebec within a 10-month period. All 3 patients had significant underlying illness, and all required surgical debridement in addition to antibiotic therapy. One of the cases fulfilled the criteria for streptococcal toxic shock–like syndrome. Group B streptococcus has been recognized as a frequent cause of serious disease in adults. It has become evident over the past decade that invasive streptococcal infections are on the increase. We speculate that group B streptococcus has recently acquired an increased ability to cause necrotizing fasciitis and suggest that this may represent the emergence of a new clinical syndrome in adults.