The electron microscopic study of 175 P. aeruginosa museum and newly isolated strains has been made and their virulence has been determined. In 32% of these strains the presence of pili has been established. However, among the museum strains those with pili have been found to constitute only 19.5%, while among newly isolated strains pili have been detected in 39.7% of cases (the difference is statistically significant at p less than 0.01). No correlation between the formation of pili and the virulence of P. aeruginosa has been established.