Sequential isolates of Pseudomonas aeruginosa from 57 infected sites in 54 patients were pyocine-typed on agar plates containing mitomycin C, by a method depending on inhibition of indicator strains by pyocines produced by the test strains. The inhibition pattern remained constant for cultures from 28 infected sites. In the remaining 29, both predominant and variant patterns could be identified. Many variant patterns differed from the predominant one only to the extent of one indicator strain. Each of two commonly occurring predominating patterns varied characteristically from the other pattern. It was suggested that minor variations in inhibition pattern reflect instability of either pyocine production or pyocine susceptibility of the indicator strains, whereas major variations in pattern signify independent infections.