Group B streptococci (GBS) are the leading cause of neonatal meningitis and sepsis worldwide. Intrapartum antibiotic prophylaxis (IAP) is the current prevention strategy given to pregnant women with confirmed vaginal GBS colonization. Due to antibiotic resistance identified in GBS, we previously developed another strategy using a bacteriophage lytic enzyme, PlyGBS, to reduce vaginal GBS colonization. In this study, various DNA mutagenesis methods were explored to produce PlyGBS mutants with increased lytic activity against GBS. Several hyperactive mutants were identified that contain only the endopeptidase domain found in the N-terminal region of PlyGBS and represent only about one-third of the wild-type PlyGBS in length. Significantly, these mutants not only have 18-28-fold increases in specific activities compared to PlyGBS, but they also have a similar activity spectrum against several streptococcal species. One of the hyperactive mutants, PlyGBS90-1, reduced the GBS colonization from >5 logs of growth per mouse to <50 colony-forming units (cfu) 4 h post treatment ( approximately 4-log reduction) using a single dose in a mouse vaginal model. A reduction in GBS colonization before delivery should significantly reduce neonatal GBS infection providing a safe alternative to IAP.