Young adults' personal mobile device use is at an all-time high. Social interaction and physical activity (PA) are generally positively associated in this population; however, the nature of this relationship is unclear when personal mobile devices (cell phones) are present. The objective of this study was to examine the social and PA behavior among college students with and without access to cell phones and a stimulus to be physically active. A between-group design that included fifty-six college students randomly exposed to one of four experimental conditions: (1) no phone/no equipment, (2) no phone/equipment, (3) phone/no equipment, or (4) phone/equipment and were observed via hidden camera. A modified System for Observing Children's Activity and Relationships during Play (SOCARP) instrument was used to quantify PA, social group size and interaction type. Significant differences were observed for PA level (p<0.001), social group size (p=0.03), physical prosocial (p=0.046) physical antisocial (p<0.001) and ignoring (p=0.037) interaction type across experimental groups with lack of access to cell phone and PA stimulus being most physically and socially active. The findings suggest that cell phones may have a detrimental impact, reducing PA engagement and impeding social interaction among young adults.