Abstract
In order to study the effect of predation on a rodent population, it was necessary to estimate accurate numbers of living rodents and of those consumed by predators. Population changes in small rodents were studied for 5 yr from 1974-1978 in Koshimizu in eastern Hokkaido, Japan. The number of rodents and their biomass were estimated as available food sources for foxes. Definite seasonal variations in the number of rodents were found but there was no outbreak. Some factors which cause erroneous estimation of rodent numbers are also discussed.