Kohlberg's developmental theory of moral reasoning hypothesizes that the socially unacceptable moral behavior of many juvenile delinquents is related to immature moral reasoning. A sizeable minority of delinquents, however, has been shown to reason at age-appropriate levels. This study examined issues related to those juvenile delinquents who were mature in moral reasoning in terms of Kohlberg's (1983) theory of sociomoral development. Incarcerated delinquents (N=43) with age-appropriate moral reasoning (Kohlberg's Stage 3) were compared with incarcerated delinquents (N=42) delayed in moral reasoning and with nondelinquents (N=35) with age-appropriate moral reasoning. All subjects completed the Sociomoral Reflection Measure, Johnson's Values Questionnaire, a shortened version of the Personal Values Questionnaire, the Quick Word Test, and a measure of socioeconomic status. In addition, the delinquent subjects also completed the Minnesota Multiphasic Personality Inventory (MMPI). The results revealed that both delinquent groups evidenced a greater acceptance of antisocial behavior and values than did nondelinquents. Both delayed and mature delinquent groups also demonstrated much psychopathology. However, mature delinquents evidenced MMPI scores reflecting more internalizing pathology than did delayed delinquents. This internalizing pathology may reflect the anxiety or guilt resulting from the strain between mature moral structure and antisocial behavior or a parallel between sociomoral structures and personality/psychopathology. (Author/NB)