The epidemiology of faecal Gram-negative bacteria was studied in 953 infants from 22 neonatal special care wards in Swedish hospitals. Two complementary methods to measure presumed nosocomial transmission of bacterial strains were used: the prevalence rate of major (M) phenotypes (strains that had colonized at least 10% of the infants in a particular ward) and the epidemic index (Ei) (which measures the probability of isolating identical strains from different infants in a ward). About 1500 strains of the most common species (Klebsiella spp., Escherichia coli and Enterobacter cloacae) were typed by the Phene Plate (PhP) biochemical fingerprinting system, and selected Klebsiella isolates were also typed by K antigen serotyping. Klebsiella spp. showed a high propensity to spread (median Ei 0.034, max 0.234), whereas E. coli (median Ei 0.006, max 0.140) and E. cloacae (median Ei 0.0, max 0.012) were more seldom involved in outbreaks. High total Eis in the wards were correlated with high cephalosporin usage (Rs = -0.56, P < 0.05). A total of about 600 biochemical phenotypes of Enterobacteriaceae strains were isolated from the 953 infants. Most of these were found in only one or a few infants, but 24 M phenotypes belonging to Klebsiella spp. (N = 19), E. coli (N = 4) and E. cloacae (N = 1) had colonized altogether 239 (25%) infants. Klebsiella M phenotypes similar or identical to each other and carrying K antigen 55 were isolated from 108 (11%) infants in eight wards, and were thus responsible for 45% of the M phenotype carrier rate in all studied infants. It is suggested that they represent a clone of Klebsiella with a high ability to multiply and spread in hospital environments.