The bacteriophage Mu A and B genes, which lie adjacent to each other and are colinear on the phage genome, encode proteins that catalyze efficient transposition of Mu DNA. We show that the molar ratio of A and B proteins is approximately 1:20 in extracts prepared after induction of cells containing a Mu lysogen or a plasmid carrying the Mu fragment that encompasses A and B. In cells harboring the cloned genes, the proteins are synthesized from a single transcript. Pulse-chase experiments demonstrate that the lower amounts of A protein are not from preferential turnover of this protein. This suggests the existence of a post-transcriptional mechanism to down-regulate A protein synthesis. From an analysis of the activity of several beta-galactosidase fusions to A protein, we infer that A protein may repress its own translation. By an agarose gel mobility-shift assay, we demonstrate that purified A protein binds specifically in vitro to its mRNA.