The relative intensity of screech owl predation on resident and transient white-footed mice was determined in a laboratory situation. Pairs of mice were exposed to the owl for 2-30 min. One of each pair had spent several days preceding the experiment in the test environment (resident), and the other had no prior experience there (transient). A significant difference in predation rates was demonstrated, transient mice being taken more frequently than residents. These results support the idea that possession of a familiar home range confers a consider- able advantage since transient mice are subject to greater danger from predation. The cause of mortality in small mammal populations is frequently unknown, although aggressive encounters, disease, starvation, temperature stress, and predation are considered potentially important (Blair, 1953; Burt, 1940; Chitty, 1958, 1960; Christian, 1961; Errington, 1946; Howard, 1949; Pearson, 1964; Pitelka, 1957). This study deals with the predator-prey relationship between the screech owl (Otus asio) and the white-footed mouse (Peromyscus leu- copus). Live-trapping studies of white-footed mice have indicated that while most mice know the terrain well and will rapidly take refuge when released, a few others move less adeptly (Burt, 1940; Metzgar, unpublished data). If the former are residents with established home ranges, and the latter are wander- ing or transient mice, transients may be preyed upon more readily than resi- dents. With the elimination of transients, the survivors would become progres- sively more difficult to capture, and predators would leave the established breeding population relatively untouched. The following laboratory experi- ment was designed to test the relative rates of predation on white-footed mice conditioned in such a way as to be analogous to residents and transients. I am indebted to Dr. F. C. Evans for his encouragement and numerous suggestions. He and Dr. Stephen T. Emlen read the manuscript. Mr. David Ligon kindly donated his screech owl, and Mr. Richard Hill gave much valu- able statistical advice. The work was done at The University of Michigan Museum of Zoology where facilities of the Bird Division were made available by Dr. H. B. Tordoff. MATERIALS AND METHODS Two adult white-footed mice, only one of which was familiar with the test area, were simultaneously exposed to a gray-phase screech owl that had been hand raised from a nestling. The mice were wild-caught in southern Michigan, toe-clipped or ear-punched for identification, and kept in the laboratory for 1 to 12 weeks in 20 X 30 cm box cages. The owl was maintained in a 1 X 1 X 2 m cage and was denied food for 24 hrs before each test.