Personality development and structure formation were studied in 60 preadolescent children (aged 9 to 13). Six major aspects of preadolescent individuality were assessed: cognitive controls, performance on the Holtzman Inkblot Test, intellectual abilities, general response to clinical testing, general response to laboratory testing, and defense mechanisms and general characteristics of ego organization. Results suggested that defenses and cognitive control organization are more fully and clearly developed at preadolescence than often assumed; that no single group of structural variables was regnant over others assessed in predicting behavior; that a number of developmental changes occur in structural organization from early to late preadolescence; and that sex differences are generally absent. Findings on all six aspects are detailed, and four case studies are presented. A bibliography cites 272 items. (DF)