The purpose of this study was to determine the validity and reliability of the Nicholas Manual Muscle Tester (MMT) as applied to ten adults with mild to moderate mental retardation between the ages of 18 to 39 years with an intelligence quotient (IQ) range of 36 to 69. Using stabilization techniques, isometric torque values were taken of knee extension and elbow flexion on separate days with the Nicholas MMT and a Cybex II dynamometer. Pearson product-moment correlations of these two instruments ranged from .64 to .76 (p less than 0.05). Based on 1 day of testing intrarater correlation coefficients for the Nicholas (MMT) ranged from .97 to .99 (p less than 0.05). In like fashion interrater generalizability coefficients were quite high (G = .97 - 98). Measurements of adults with mental retardation taken by two experienced evaluators with a Nicholas MMT seem to evaluate the component-isometric strength. Highly reliable measurements with stabilization techniques were obtained with this instrument.