During the period 1979-1991, Salmonella Typhimurium DT 204c was the cause of a major epidemic of salmonellosis in calves in the UK. Plasmid profile analysis of DT 204c isolates from England and Wales commenced in 1986 and isolates from all subsequent incidents were examined by this technique. Forty-three different plasmid profile types (PPTs) were detected, of which the commonest, designated type E, constituted 44.6-80.2% of the annual incidents during the study period. Some PPTs, e.g., F and P, were detected throughout most years of the study, whereas PPTs O and 6 persisted for short periods. Until 1984, most isolates were resistant to neomycin, but the subsequent predominant PP type E was sensitive to this antibacterial agent. It was concluded that during the epidemic there was an evolution of new genotypes, of which only some persisted; again, antibacterial resistance genes may be acquired or lost. The study demonstrated the value of PP typing for epidemiological studies.