This study was done to determine the extent of tree damage (primarily bark‐stripping) caused by blue monkeys(Cerco‐pithecus mitis)in the Sokoine University of Agriculture Training Forest in the Mount Meru Forest Plantations. The impact of monkeys was extensive; 88.7% of the pine (Pinus patula)amd 79.5% of the cypress(Cupressus lusitanica)trees were damaged. Intermediate pine trees were damaged more than dominant trees, whereas dominant cypress trees incurred the most damage. Suppressed trees of both species were least damaged. For all types of trees the most severe damage occurred at the middle and top.of the tree trunks. Damage was low in the rain season and increased in the dry season, peaking in June and July when 22% of the trees were damaged. Differences in the ease of bark peeling and nutritional elements in the bark of the two species, and shortages of natural foods, are discussed as possible explanations for the variations in damage between seasons, tree species and types of trees.