During a recent study carried out at an isolated site in the Colorado mountains, a C5 alcohol, 2-methyl-3-buten-2-ol, was found to be the most abundant volatile organic compound of biogenic origin present in the atmosphere. This finding, if generally characteristic of the natural chemical species present in the atmosphere in forested areas, has important implications. First, the presence in large quantities of a reactive chemical compound at these high levels can significantly influence the local atmospheric chemistry. Secondly, this compound, although previously identified as a pheromone for Ips typographus (spruce bark beetle), an insect predator responsible for major forest die-backs in this region, is strongly correlated with isoprene. Since isoprene is known to be emitted by the local vegetation, the observed 2-methyl-3-buten-2-ol appears also to have a vegetative rather than entomological source. ¿ American Geophysical Union 1993