The Trail, a competition open to all types of runners, takes place in a natural environment and its total route does not exceed 20% of asphalt routes. Trail runners tend to spend most of the race below aerobic threshold intensities. Among the genetic factors that determine aerobic performance, the Ins/Del polymorphism of de ACE gene, is considered a determining marker of maximum oxygen consumption (VO2max). PURPOSE: To evaluate the differences in the maximum oxygen consumption according to genotypes of the Ins/Del polymorphism of the ACE gene in trail runners of Valdivia. METHODS: Analytical cross-sectional study. 41 Trail running from the city of Valdivia were evaluated and the Trail test was applied to them, consisting in a 3-min warm-up at 8 km/h, increasing to 10 km/h at the beginning of the test, then increasing continuously 0,5 km/h per minute, as the inclination, increasing 1% per minute and starting at 0,5%, until volitional exhaustion. During the test, breath by breath analysis for oxygen consumption using a Jaeger Oxycon Mobile ergospirometer was performed, with 11 breaths average and 10 second recording time. In addition, the frequency of appearance of alleles and genotype of the Ins/Del polymorphism of the ACE gene was determined. The variables were compared by ANOVA or t-test, with a significance level of p <0.05. RESULTS: The genotypic distribution was: Ins/Ins 27%; Ins/Del 51%; Del/Del 22%. The frequency of the allele Ins 52% and Del 48%. The mean VO2max was 50.29±7.84 ml/Kg/min. No statistically significant differences were found when grouped by genotype (p = 0.810), except in oxygen pulse (p <0.009) when grouped by dominant Del allele. CONCLUSIONS: Differences in maximum oxygen consumption were evaluated according to Ins/Del polymorphism of the ACE gene, however, for this population of amateur runners, the polymorphism would not be a determinant of aerobic performance. It is necessary to perform field-based evaluations to assess all the components related to the specific physiological demands