Streptococcus agalactiae is the leading cause of bacterial sepsis and meningitis in neonates and also the causative agent of different serious infections in immunocompromised adults. The wide range of diseases that are caused by S. agalactiae suggests regulatory mechanisms that control the formation of specific virulence factors in these bacteria. The present study describes a gene from S. agalactiae, designated rogB, encoding a protein with significant similarity to members of the RofA-like protein (RALP) family of transcriptional regulators. Disruption of the rogB gene in the genome of S. agalactiae resulted in mutant strain RGB1, which was impaired in its ability to bind to fibrinogen and fibronectin. Mutant RGB1 also exhibited a reduced adherence to human epithelial cells but did not show an altered invasion of eukaryotic cells. By real-time PCR analysis, mutant RGB1 revealed an increased expression of the cpsA gene, encoding a regulator of capsule gene expression. However, strain RGB1 exhibited a reduced expression of the rogB gene and of two adjacent genes, encoding putative virulence factors in S. agalactiae. Furthermore, mutant RGB1 was impaired in the expression of the fbsA gene, coding for a fibrinogen receptor from S. agalactiae. The altered gene expression in mutant RGB1 could be restored by plasmid-mediated expression of rogB, confirming a RogB deficiency as the cause for the observed changes in virulence gene expression in S. agalactiae. Reporter gene studies with a promotorless luciferase gene fused to fbsA allowed a growth-dependent analysis of fbsA expression in S. agalactiae. These reporter gene studies also suggest that RogB exerts a positive effect on fbsA expression in S. agalactiae.