Several bark beetles transform toxic monoterpens secreted by their host to aggregation pheromones. Although Dendroctonus valens is attracted by R-(+)-α-pinene and S-(-)-β-pinene and transform it, no aggregation pheromone has been identified for this species. Histological and ultrastructural studies indicate that this biotransformation take place in epithelial cells of alimentary canal, suggesting that processing of this monoterpens could be associated to detoxification processes. One of the most common enzymes involved in detoxification of xenobiotics is carboxylesterases. The aim of this study is to measure esterase activity and identify different non-specific esterases by histochemistry in different regions of alimentary canal of D. valens exposed to α-pinene. Adult insects were exposed to R-(+)-α-pinene, S-(-)-α-pinene and S-(-)-β-pinene during 12 and 24 hrs. After this, we obtained histological sections of alimentary canal. We did histochemical reaction to detect esterase activity. In the other hand, we use different esterase inhibitors to identify which non–specific esterase are in the different tissues of alimentary canal. We observed more esterase activity in foregut, anterior and posterior midgut cells than in hindgut cells. Esterase activity was greater with R-(+)-α-pinene and S-(-)-α-pinene than with S-(-)-β-pinene and no exposed insects. Also, we identified greater carboxyesterase activity in foregut and midgut than arylesterase and acetylesterase activity.