Suspensions obtained from five Pseudomonas aeruginosa pyocinogenic strains showed inhibitory and variable activity against bacterial strains belonging to the Nocardiaceae, Micrococcaceae, Neisseriaceae, Streptococcaceae, Vibrionaceae, Enterobacteriaceae, and Pseudomonadaceae families. Under special conditions, the same pyocinogenic P. aeruginosa strain can be affected by it own suspensions. These pyocinlike particles could be considered as a regulatory factor acting on the rate and size of the population growth.