The concept of teaching moral character through sport has recently received a resurgence of importance (Sage, 1998). This study attempted to reduce negative sport behavior and increase positive sport behavior during a season of youth basketball using Hellison's Social Development Model (1985). A multiple baseline across behaviors with multiple probes design was used. Four male athletes, 10 to 12 years of age, were given parental permission to participate. Positive sport behaviors were defined as physical, verbal, or nonverbal positive social interaction such as giving another player encouragement, support, assistance, or positive feedback. Negative sport behaviors were defined as physical, verbal, or nonverbal negative social interaction such as those conveyed through complaining, disrespectful expression to other athletes or coach, or teasing other athletes. Data were collected by direct observation of the target athletes once weekly for twelve weeks during practice and games. During baseline conditions athletes displayed higher rates of negative sport behavior than positive sport behavior. Using Hellison’s Social Development Model, there was a decrease in negative sport behavior for all the athletes and an increase in positive sport behavior for three of the four athletes during practice. Similarly during game conditions there was an increase in positive sport behavior for all four athletes and a decrease in negative sport behavior for three of the athletes. The fourth athlete’s negative sport behavior during game conditions was unchanged from baseline measures. Data were also collected on three of the athletes during a subsequent sport season in which they played immediately following the basketball season in which the intervention was used. During the maintenance period, two athletes increased their positive sport behavior threefold, while the other more than doubled his positive sport behavior during game conditions. In addition, these athletes further decreased their negative sport behavior. This study supports the use of Hellison’s Social Development Model as a successful intervention for decreasing negative sport behavior and increasing positive sport behavior. Graphic representation of the data for each athlete shows a convergence of the negative and positive sport behavior data points over time (from baseline to maintenance). During baseline conditions higher rates of negative behavior compared to lower rates of positive behavior were exhibited, but by mid-season the rates of negative and positive sport behaviors had reversed. From baseline to maintenance there was a distinct downward trend in negative sport behavior and an upward trend in positive sport behavior for the athletes.