A mountain pine beetle infestation has been active in the St. Regis area since 1964. Heaviest tree mortality occurred during the period 1969-70. The infestation has continued to decline since 1970. Over 2 million board feet of lodgepole and ponderosa pine were killed during the outbreak. Overstocking resulted in slow growth which probably promoted a general weakening of the stand, making some trees more susceptible to attack by mountain pine beetle. Two factors that might be contributing to the decline of the outbreak are: 1. The influence of Ips spp. drying out the cambial area in infested trees. 2. The average size of trees being attacked is less than 10 inches d.b.h., resulting in fewer beetles emerging from infested trees than parent adults that attacked.