The seg-3 mutant Escherichia coli does not support the maintenance of mini-F plasmid at 42 degrees C. We cloned the chromosomal DNA segment of the wild-type strain W3110 that complements the Seg- phenotype of this mutant. Cleavage mapping of this segment showed that it was derived from the 76-min region of the E. coli chromosome map. Complementation tests using plasmids carrying subcloned DNA segments suggested that the seg-3 mutant carried two mutations that additively affected the maintenance of mini-F plasmid; one was in the ugpA gene and the other was presumably in the rpoH gene. We generated a disrupted ugpA null mutant and found that the mini-F plasmid was unstable in this ugpA null mutant even at 30 degrees C. This suggests that the ugpA gene product is required for the stable maintenance of mini-F plasmid.