Infections with Gram-negative bacilli have affected critically ill newborns throughout the era of modern neonatal intensive care. Selected topics regarding the acquisition of Gram-negative bacteria, particularly those expressing antibiotic resistance, in the neonatal intensive care unit (NICU) are reviewed. Some data suggest that the recent introduction of intrapartum antibiotic prophylaxis programs, through which selected women are administered penicillin or ampicillin during labor to prevent vertically transmitted Group B streptococcal disease, has increased the incidence and resistance of early onset Gram-negative bacillary infection in the newborn over the past 5 years. Additional data suggest that late onset, nosocomially-acquired Gram-negative bacillary infections also are increasing among critically-ill newborns. The seminal event in Gram-negative bacterial infection in the hospitalized infant is colonization of the gastrointestinal tract and nasopharynx. Newborns admitted to the intensive care unit acquire hospital-associated Gram-negative bacterial colonization throughout their NICU stay, presumably transmitted from the environmental flora via the hands of caregivers. Colonization specifically by antibiotic-resistant bacilli follows a similar pattern, suggesting that such bacteria make up part of the modern NICU ecology and are acquired by infants similar to susceptible microorganisms. Althou-gh some clinically-undetected cross-transmission of resistant bacilli occurs during non-outbreak periods, most colonizing antibiotic-resistant bacilli are unique to each infant. The role of antibiotic exposure on the acquisition of antibiotic resistant bacilli in the intensive care nursery is difficult to calculate given the covariance of such exposure to other markers of severe disease. Experience has demonstrated, however, that use of the aminoglycosides is infrequently associated with emergence of resistance in the newborn, whereas the use of higher-generation cephalosporins may be associated with the rapid appearance of bacilli resistant to betha-lactams.