Skinfold measurements were taken at 6 selected sites on 114 participants at the 1968 U. S. Olympic Marathon Trial. For the purpose of comparison, similar measurements were taken on 92 normally active men and 63 age-matched graduate students and faculty men. Body density and fat were estimated by 3 separate equations selected after a thorough review of the literature. An average of the 3 equations resulted in a body density estimate of 1.084 for the marathoners as compared to 1.071 and 1.062 for college men and faculty, respectively. It was concluded that marathon runners possess about 7.5% body fat or 5% less fat than the college men and nearly 9% less than the faculty group.