A total of three hundred and ninety nine isolates of Salmonella enterica were recovered from 3106 cloacal swab cultures, chicken carcasses and environmental samples taken from a broiler breeding farm over six successive generations. S. enterica serovars Berta (S. Berta), 4,12: b:—, Taksony, Binza and Typhimurium were identified. One hundred and seventy one strains of S. Berta were analysed by plasmid profiling to determine the epidemiological relationships between isolates from each generation and from each house section of the production unit. All isolates contained plasmids; plasmids of eleven different molecular weights were identified. Eighteen different plasmid profiles were identified throughout the observation period. An increase in the diversity of plasmid profiles in each successive generation, the restriction of particular plasmids to particular houses and their persistence within that house were taken to indicate that contamination within the unit was due to persistence and cross-infection rather than to a reintroduction of serovar Berta into new generations of stock.