Expression of the bacteriophage Mu mom gene is subject to posttranscriptional regulation by the phage com gene product. We have used mom-lacZ translational fusion genes to define the sequence requirements for stimulation of mom expression by Com. We show that the mom translation initiation region (TIR) is inactive in the absence of Com. We suggest that this repressed state is due to mRNA secondary structure in the TIR, since a deletion that destabilizes a stem-loop structure in the TIR results in high levels of Com-independent translation. We identify sequences on the mRNA, adjacent to the stem and loop, that are required for stimulation by Com. We propose that Com acts to stimulate initiation of translation by relieving the structural repression of the mom TIR. Indirect evidence is presented suggesting that Com binds to a site in the TIR.