We have investigated the presence of plasmids in 96 Legionella pneumophila serogroup 1 strains isolated from clinical and environmental samples from the Valencia area. Our results show a higher incidence of a 83 MD plasmid present in clinical as well as in environmental isolates, and a 120 MD plasmid with lower incidence and observed exclusively in strains isolated from one area. The variability in plasmid detection from L. pneumophila and the effect on it in a hospital after disinfection treatment is considered.