SUMMARY: The value of a line of traps as a measure of relative abundance of small mammals was studied by field trials on Peromyscus leucopus populations. Comparisons were made between the numbers of mice captured by a line of live traps and the numbers captured in intensive live trapping of a larger area surrounding the line. Trials were made in bottomland woods where mice were numerous and in upland woods where mice were less common. It was found that wood mice living in upland woods had significantly larger cruising ranges than those living in bottomland woods. Consequently, a line of traps in the bottomlands captured mice from a smaller surrounding territory than in the uplands. Therefore, comparisons of relative size of the mouse population in these two areas on the basis of line-trapping showed an erroneously large number for the upland woods. As a result of these trials and the studies of other workers, it is concluded that lines of traps are not fully reliable means of measuring relative abundance of small mammals.