Bacillus subtilis and related bacilli produce a posttranslationally modified oligopeptide, the ComX pheromone, that stimulates natural genetic competence controlled by quorum sensing. The ComX(RO-C-2) pheromone from strain RO-C-2 must be modified with a farnesyl group on the Trp residue, but the precise structure is not known. Here we report the precise nature of posttranslational farnesylation of ComX(RO-C-2) pheromone on the Trp residue, resulting in the formation of a tricyclic structure. The ComX(168) pheromone, produced by the standard laboratory strain used in the study of B. subtilis, is also posttranslationally farnesylated according to phylogenetic resemblance.