Streptococcus pseudoporcinus, a beta-hemolytic microorganism first isolated from the female gastrourinary tract in 2006, cross-reacts with serogrouping kits for group B Streptococcus (GBS) and could be misidentified in the laboratory. The epidemiologic characteristics of this species have not been reported previously, but this organism is thought to be rare. Paired vaginal and rectal samples were collected from 663 nonpregnant women enrolled in a phase II clinical vaccine trial of a GBS type III capsular polysaccharide-protein conjugate vaccine, and isolates initially identified as S. pseudoporcinus were collected for further testing. A total of 120 isolates of S. pseudoporcinus were recovered from 36 unique individuals with 5.4% of 663 women having this organism recovered at least once during follow-up. All of these isolates cross-reacted with a commercially available GBS serogrouping kit. Women colonized with isolates confirmed as S. pseudoporcinus by genotypic and phenotypic methodologies were compared to women who were not colonized to determine whether there were any significant factors associated with acquisition of S. pseudoporcinus. Acquisition of S. pseudoporcinus vaginally and/or rectally was 36 per 846.0 women-years of follow-up for an annual incidence of 4 per 100 woman-years of follow-up. Acquisition of S. pseudoporcinus was independently associated with black women, being 30 to 40 years of age, recent Trichomonas vaginalis infection, primary or recurrent genital herpes, having bacterial vaginosis by Nugent criteria, and having had two or more male sexual partners since the last visit. This study suggests that S. pseudoporcinus is not rare, especially among black women, and could be misidentified as GBS.Streptococcus pseudoporcinus is a facultative Gram-positive cocci usually characterized by a large zone of beta-hemolysis. It was initially thought to be Streptococcus porcinus, an organism in serological groups E, P, U, and V and found in the upper respiratory and genital tracts of swine (3), but in 2005, Duarte et al. demonstrated that strains of S. porcinus isolated from humans had unique pulsed-field gel electrophoresis profiles of the chromosomal DNA compared to the nonhuman strains and the majority of the human isolates belonged to serogroup NG1 (5). In 2006, Bekal et al. using 16S rRNA gene sequencing demonstrated that the organism found in humans had biochemical characteristics similar to those of S. porcinus but were genetically unique and proposed the name Streptococcus pseudoporcinus (1). Thompson and Facklam from the Centers for Disease Control and Prevention tested human strains of S. porcinus (serogroup NG1) with the group B reagents of 12 commercial Streptococcus test kits and reported that all of the kits evaluated cross-reacted with one or more of the isolates (17). Because S. pseudoporcinus cross-reacts with serogrouping kits for group B Streptococcus (GBS), S. pseudoporcinus may be misidentified as GBS in clinical settings.S. pseudoporcinus has been isolated from multiple specimen ...