Knowledge of mating system characteristics can elucidate forces driving sexual selection. In male pitvipers, both male movement tactics and body size are predicted to be important determinants of reproductive success. We used radio telemetry to monitor free-ranging Sistrurus catenatus (Eastern Massasauga) from 2000 through 2002 to determine whether male movement tactics and body size affect mate acquisition. Reproductive behaviors peaked in late July to early August. Females were accompanied by multiple males per season (up to seven); however, male mate acquisition varied considerably with only three of 17 (18%) males located more than one female during a single mating season. During the mating season, male mean daily distance moved (21.8 m) and movement frequency (77%) were higher than during the nonmating season (13.3 m, 63%). Male movement rate and body mass were positively related to the number of females acquired, and heavier males were observed accompanying females as the mating season progressed. Our results indicate that both movement tactics and body size are important in the mating system of S. catenatus; however, direct measures of reproductive success will be necessary before assessing the intensity of sexual selection.