SUMMARYAlthoughStreptococcus agalactiaehas emerged as an important cause of invasive disease, relatively little is known regarding the genetic basis of virulence of this organism. Three novel genes with characteristics suggesting a role in virulence were identified via comparison of sequenced genomes ofS. agalactiae. The presence of these genes and of the previously identified genesbac,bca,rib, andspb1was determined, and isolates were assigned a binary genetic signature. It was found that isolates containingspb1, previously suggested to be limited to serotype III-3, were represented by 18 different genetic signatures and several serotypes, and that the presence of bothsbp1andribwas more predictive of invasive disease thanspb1alone. Additionally,bac-positive isolates, reported to be genetically homogeneous, were represented by 14 different genetic signatures. Finally, the majority of serotype V isolates examined contained zero or only one of the genes tested, suggesting that much remains undiscovered regarding important virulence factors in isolates of this serotype.