We have isolated a high copy number mutant of plasmid pSC101 which is maintained at a level 4 times higher than that of the wild type. The mutation is a single base change that maps in codon 93 of the initiation protein RepA. We find that the mutation relaxes the autoregulation of the protein but increases its affinity for the repeated sequences in the origin. The wild type and the mutant repA genes are co-dominant and the mutated protein acts in trans even in the presence of the wild type protein. Co-maintenance of the two types of plasmids results in an intermediate copy number. Computer simulation indicates that simple models can explain the behaviour of the two plasmids.