Genetic drift in an outbred stock of CFLP mice was monitored for 6 years using genetic polymorphism for glucose phosphate isomerase (GPI) and the shape of the mandible as genetic markers. Statistically significant drift was found in both characters. The cause of this drift was not determined, though it could have arisen as a result of genetic sampling during hysterectomy derivation. The use of inbred strains is recommended on theoretical grounds of long-term genetic constancy is needed.