The gene coding for the 33,000 dalton protein (B protein) of the mini-F plasmid has been mapped by minicell polypeptide analysis to 47.3-48.2 Fkb. Transcription of the gene is initiated near 47.3 Fkb. Gene B mutants overproduce the 42,000 dalton protein (A protein) of mini-F. Insertions of Tn 5 in the B gene and in the incD region cause instability of plasmid inheritance. Mini-F plasmids delected for part of gene B are not maintained in the absence of the incD region. B protein and the incD region appear to interact in promoting mini-F maintenance.