Survival, colonization and activity of Pseudomonas syringae bacteria inoculated onto the leaf surface of the common bean (Phaseolus vulgaris) was studied. Inoculated Ps. syringae cells shortened by half their size in 100% humidity and by an average of one fifth in 40-60% humidity. The respiring portion of the population, measured by the formation of 5-cyano-2,3-ditolyl tetrazolium chloride (CTC)-formazan crystals, decreased more in 40-60% humidity than in 100% humidity. In scanning electron micrographs, the bacterial cells on leaf surfaces were seen embedded in a mucoid matrix. Intraspecies conjugation of plasmid RP1 also occurred in 40-60% humidity conditions. The portion of transconjugants temporally rose higher than the same portion in 100% humidity conditions. Therefore, although only a small proportion of the inoculated cells remained active on the leaf surface in 40-60% humidity, a relatively high rate of conjugation was still seen. Gene spreading was thus efficient on the leaf surface also when conditions did not allow bacterial population growth.