The Self-Motivation Inventory (SMI) and four subscales from the Philosophies of Human Nature were given to 50 male ultramarathoners who were matched for age and sex with runners who had never raced any distance longer than 10 miles, and with a group of nonrunners. No significant personality differences were obtained among the three groups. Experienced ultramarathoners were Significantly. more skeptical about the goodness of human nature than inexperienced ones.