Estimates of numbers were made five times a year over the period from [1975][1976][1977][1978][1979][1980] in a population of Clethrionomys glareolus (Schreber, 1780) living on Crabapple Island in northeastern Poland, using the CMR method. Parallel to the above the standing crops of herb biomass v/ere estimated in four forest associations covering the island. A total of 1812 individuals and 22,959 captures were recorded. It was found that fluctuations in the number of individuals and their captures coincide in all the associations, whereas the latter differ in respect of the average number of individuals recorded there and the number of their captures. The largest number of individuals was found in the camp and highly productive herb layer Circaeo-Alnetum and the smallest in the Tilio-Carpinetum typicum association situated on the highest and dryest ground. The largest number of captures was obtained in Circaeo-Alnetum and Tilio-Carpinetum stachyetosum silvaticae associations. The pictures obtained for the different categories of individuals (males, females and among them mature and immature animals) differed, which points to their different habitat requirements. A proportionate increase in the number of captures does not always accompany increase in the number of individuals of a given category: in some associations in which, although a large number of individuals of the given category is observed, the number of captures is small, and vice versa. This indicates that the term habitat optimum can be taken to mean several things. A discussion is given of the importance of determining the habitat requirements of different elements of popopulation structure in order to define their role in the ecosystem. Attention was also given to the methodical aspect, connected with estimate of numbers by means of snap-traps or the use of the trap-nights index.