Rodent densities were estimated on Mount Usu, northern Japan, 17 years after the 1977–1978 volcanic eruptions. The summit area was completely deforested by the eruptions, and was covered with thick tephra. When the survey was performed, two plant communities developed. One was a forest dominated by Populus maximowiczii, and another was a grassland with a canopy of the large perennial, Polygonum sachalinense. In each plant community, we estimated the abundance of rodents by Sherman traps. The abundance was 5 Clethrionomys rutocanus bedfordiae/100 traps/night in the grassland, and 0 in the forest. This implied that a generalist, C. rutocanus bedfordiae became established in the grassland, probably because of plant community structure, including litter deposits, rather than food resources. The results compared with those from Mount St. Helens suggested that rodent immigration was greatly determined by the scales of eruptions and recovery patterns of plant communities.