In the laboratory, Peromyscus bearing a single Cuterebra larva are no more vulnerable to weasel predation than are uninfected control mice, and may be taken less often under certain conditions. Mice bearing two or more larvae appear to be more vulnerable than either controls or singly infected mice. Their increased vulnerability probably results from their failure to use arboreal pathways. Decreased activity may be responsible for the relative advantage of singly infected mice. Previous reports of higher survivorship among mice with a single bot parasite than among uninfected mice, and of lower survivorship among multiply infected mice, may result in part from differential predation rates.