Abstract Sixteen strains of Legionella pneumophila serogroup 1 isolated from patients and the environment at Santa Cruz Hospital were examined using four typing methods. Monoclonal antibodies showed that all the isolates belonged to subgroup Pontiac and mainly to subtype Allentown. With multilocus enzyme electrophoresis nine subtypes (ETs) were revealed; each strain had the same profile with electrophoresis of soluble proteins, and plasmid analysis revealed that only two of the strains studied contained plasmids. Among these methods, multilocus enzyme electrophoresis was the most discriminative as a single epidemiological marker. Problems concerning the microbiological examination of environmental specimens and correlation with clinical strains confirmed the difficulty of investigating an outbreak source of legionellosis and the need for careful evaluation of the results.