Much of the small mammal trapping done at the present time is carried out in order to estimate the number of animals present. In the following discussion are presented two methods for interpretation of trapping results. Small mammals are taken by two fundamentally different types of trapping, first, in experiments where the animals are marked and released into the population from which they were captured, and second, in experiments where the animals are removed as captured. The ecological advantages of mark-and-release trapping have been recognized by numerous investigators. Removal trapping, on the other hand, has practical advantages, and numerous sets of data from such studies are now available.

The two types of trapping records are discussed separately, one method for estimating the population being outlined for each of the two types. There are available for use several approaches based upon probability of capture, providing certain assumptions are made. The two methods here discussed have been applied with apparent success to numerous records of trapping the meadow vole (Microtus pennsylvanicus pennsylvanicus Ord) and to several other sets of data. Appreciation is expressed to Professor Leo Katz of Michigan State College for helpful suggestions, and especially for pointing out the importance of assuming that the regression line passes through the origin in the method offered for use with mark-and-release trapping.