Background: Type I human immunodeficiency virus (HIV) is characterized by a great genetic variability. There are three groups of virus throughout the world: O, N and M. Group M is responsible for AIDS pandemic and is subdivided in 9 genetic subtypes. Most viral strains in South America are subtype B. Aim: To determine the frequency of HIV subtypes in Chilean patients. Material and methods: Genetic analysis of C2-V3-C3 region of the gene env in HIV strains coming from 77 Chilean subjects infected by different means. DNA heteroduplex mobility assay was used to determine HIV subtypes. Results: Sixty eight cases were infected with subtype B (88.3%) and nine cases were infected with subtype F (11.7%). Conclusions: Subtype B is the predominant HIV in Chile, but subtype F is also present (Rev Méd Chile 2003; 131: 711-8)