Sixteen male delinquent mentally retarded subjects, clients of a clinic for the retarded, were classified into four groups. Social maturity, life histories, and delinquent behaviors were evaluated. Subjects were interviewed to establish their adjustment to work and attitudes towards the law. Group 1 delinquents, those whose delinquencies occurred in direct relation to their rigid and limited adaptive behavior related to their mental retardation, generally were of low intellectual level (under 40 IQ (intelligence quotient) by Binet type). This low intellectual level may limit their adjustment to the standard of socialization and control values. Their delinquencies, such as arson, are seen as primitive responses. Group 2 delinquents, of a slightly higher intellectual level and social maturity than other groups, are apt to set goals at the same level as normal persons and to claim the same treatment. Their delinquencies have almost the same variation as those of the normal intelligence, yet their delinquent techniques are poorer. Group 3, whose adaptive levels were lower than those of group 2, could not adapt to their work. This failure contributed to their delinquency. Delinquents in group 4 failed to learn a consistent standard of behavior because the standards of training or socializing in their families and living environments were confused or inadequate. The study indicates that the formation of delinquency can be understood by searching for a relationship between two dimensions, 'conformity to criminal environment-perseverance of personal tendency,' and 'maladjustment to socialization-adjustment to antisocial values.' However, these case studies contained a very small number of mentally retarded delinquents. Although delinquency of the mentally retarded shows the mechanism of delinquent behavior in a simple pattern, it may be erroneous to apply this conclusion to delinquents of normal intelligence. Two references and tabular data are appended. (Author abstract modified)