A SUPERFICIAL examination of Pacific mud turtles, Clemmys marmorata (Baird and Girard), from various localities readily reveals difences in external characters. The purpose of this study was to ascertain whether these differences are geographic or merely individual. Correlation of external characters with age or sex must be understood before further analysis is attempted. Most of the specimens examined in the course of the present study are in the Museum of Vertebrate Zoology of the University of California; others were from the California Academy of Sciences, Stanford University, the San Diego Society of Natural History, the private collection of L. M. Klauber, and the College of Puget Sound. Acknowledgment is made to those in charge of these collections who made the material available. In all, 158 specimens were studied; of these 138 were from California, 9 from Oregon, 6 from Lower California, 4 from Washington, and 1 from Idaho. Measurements were made to the nearest millimeter by means of 200 mm. calipers. The length of the carapace and of the plastron was taken as the distance between the mid-anterior and mid-posterior margins of each. The external characters of specimens were compared with those given by Van Denburgh (1922: 974-976), whose description was found to be the most recent and complete. Particular attention was paid to characters that differed from his description and to those indicated as variable.