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Children (mean CA = 12 years) with mental retardation and adults without mental retardation classified tetrads of stimuli that could be grouped according to identities on separate dimensions or according to overall similarity. When color, size, and line orientation were varied (Experiment 1), both groups used separate dimensions for classification. When hue, saturation, and brightness varied (Experiment 2), both groups used overall similarity for classification. Test-retest correlations showed that the predominant classifications were reliable across 1-week testing intervals. Results support Garner's distinction between separable and integral stimulus structure but do not support an integral-to-separable shift in perceptual development. Comparisons of the tetrad task used in the present study and triads used in previous work are discussed as well as implications of the present data for developmental theories of perceptual classification and processing in children with mental retardation.