A sudden increase in the incidence of ampicillin resistance was observed among Salmonella species isolated within New Zealand in 1973–4. This increase was due mainly to the appearance and proliferation of Salmonella newington and Salmonella anatum serotypes resistant to ampicillin. The plasmid complements of 14 ampicillin-resistant S. newington and S. anatum isolates obtained from widely separated geographical areas within New Zealand between 1973 and 1974 were characterized by agarose gel electrophoresis. Each contained one or more plasmids ranging in molecular weight from 1·4 to 42 Mdal. Eleven isolates contained a self-transmissible plasmid of 33 Mdal which encoded resistance to ampicillin. After transfer to Escherichia coli, the 33 Mdal R-plasmids from each of these isolates were shown to be identical by restriction endonuclease analysis. The remaining three strains contained ampicillin R-plasmids having molecular weights of 35, 37·5 and 42 Mdal. These plasmids were shown by restriction endonuclease analysis to be related to the 33 Mdal R-plasmid. We conclude that the 33 Mdal plasmid and its derivatives were responsible for the increase in the incidence of ampicillin-resistant S. newington and S. anatum serotypes among the total Salmonella population.