Mass-release of Cryphalus fulvus beetles as a vector of the entomogenous fungus Beauveria bassiana to control Monochamus alternatus larvae was done at the coastal forest of Akita Pref. in the summers of 1995 and 1996 using an improved mass-release system. In each year, over 20, 000 individuals of C. fulvus were released by an application device in which B. bassiana conidia adhered to the surface of the beetle at the time of release. Infection of M. alternatus larvae by B. bassiana was investigated using standing logs in four directions that were infested with M. alternatus. Average mortality of M. alternatus larvae before overwintering was 1.3% in 1995 and 13.9% in 1996, and the mortality in 1995 rose to 11.5% by the following summer. As the distance from the application device to the infested trees increased, the mortality of M. alternatus larvae decreased. It was not clear whether mortality was different in different directions. Mortality was relatively higher in the middle of the trunk than in the upper and lower parts of the trunk. These results seemed to be affected by the flight abilities and infestation behaviors of C. fulvus, which prefers thin bark. Further improvements of this method to control M. alternatus in natural stands are needed.