The penicillin binding proteins (PBSs) of Bacteroides fragilis, a clinically important Gram-negative rod, were studied. Four PBPs were detected by polyacrylamide gel electrophoresis/ fluorography, PBP 4 (molecular weight, 35, 000) being a minor PBP. The PBP pattern was thus different from that of the Enterobacteria and Pseudomonads. Antibacterial activity of β-lactam antibiotics was associated with binding to PBP 1 (molecular weight, 100, 000), 2 (molecular weight, 86, 000) and 3 (molecular weight, 68, 000). Binding to PBP 2 was associated with filamentation while binding to PBP 1 resulted in cell lysis.