The present study evaluated the effects of a reinforcement based aerobic and resistance exercise program on three obese men with mental retardation and below average fitness levels. A multiple-baseline-across-subjects design was employed to evaluate treatment effectiveness and retention of treatment effects on five dependent measures: body weight, percent body fat (body composition), oxygen consumption (predicted max V̇O₂ in ml/kg/min), composite isometric strength (in kg of force), and work productivity (pieces of work completed). Subjects improved during treatment from their baseline scores on cardiovascular fitness, strength, and work productivity measurements (p<.05); however, retention of gains made during treatment were inconsistent and the data that indicated subjects’ scores were regressing back toward baseline measurements. There were no significant differences for body weight and percent body fat measurements for treatment and retention phases (p>.05). Results indicated that adults with mental retardation respond to a progressive exercise program in much the same manner as their nonretarded peers and that such an exercise program can facilitate job performance.