Host acquisition of a new microbial species can readily perturb the dynamics of preexisting microbial associations. Molecular cross talk between microbial associates may be necessary for efficient resource allocation and enhanced survival. Classic examples involve quorum sensing (QS), which detects population densities and is both used and coopted to control expression of bacterial genes, including host adaptation factors. We report that a 56-amino-acid repressor protein made by the resident psyllid endosymbiont
Wolbachia
can enter cells of
Liberibacter crescens
, a cultured proxy for the uncultured psyllid endosymbiont “
Ca
. Liberibacter asiaticus” and repress “
Ca
. Liberibacter asiaticus” phage lytic cycle genes. Such repression in “
Ca
. Liberibacter asiaticus” may be critical to survival of both endosymbionts, since phage-mediated lysis would likely breach the immunogenic threshold of the psyllid, invoking a systemic and nonspecific innate immune reaction.