Amniotic membranes collected after both cesarean and vaginal deliveries were inoculated with group B streptococci (GBS) in this in vitro study. Transmission electron microscopic examination of segments of uninoculated control amniotic membranes revealed compact, well-ordered, clearly defined layers of collagen fibrils. Examination by both scanning and transmission electron microscopy of amniotic membrane segments inoculated in vitro with group B streptococci revealed bacterial attachment to the membrane surface and migration through the membrane accompanied by disordered collagen fibril layers. Degradation of the collagen fibrils during bacterial invasion may cause weakening of the amniotic membranes and thus be a contributing factor in cases of premature rupture of membranes associated with group B streptococcal colonization of the mother.