The population structure and transmission of Escherichia coli in a small group of individuals isolated for 26 weeks on an Antarctic base were studied by multilocus electrophoresis of eight enzymes and plasmid analysis. Two hundred and sixty-nine strains were isolated. They were grouped into 60 allozyme types (ETs). Half of these ETs were only isolated once; others were repeatedly isolated from single subjects. Eleven were found in more than one subject and the pattern of the occurrence of some of them was considered to provide evidence of their spread from subject to subject.