Strains of Pseudomonas aeruginosa (P. pyocyanea) were examined for pyocin production by means of a standard pyocin-typing technique and also a modified method which depends on the ability of some pyocins to pass through a cellulose acetate membrane. The results suggested that strains could produce ( I ) pyocins incapable of spontaneous passage through the membrane, (2) pyocins capable of passage through the membrane, (3) both kinds of pyocin simultaneously. Electron microscopy revealed that pyocins in the first circumstance included contractile pyocins and those in the second included flexuous, rod-like particles. P. aeruginosa 430 produced both contractile and flexuous pyocins simultaneously. The pyocins could be separated by absorption with sensitive bacteria or by gel filtration. The two pyocins differed serologically and in their range of inhibitory activity against strains of P. aeruginosa.