The entire age cohorts of 11- and 14-year-old adolescents and all 17-year-olds who could be identified from school registers from five mid-sized cities in Western Finland (n=2,940) were included in the study. A total of 2,594 (1,307 girls, 1,287 boys) eventually participated in the study. Evaluation of theft, lying, and fighting was measured with a 21-item questionnaire. All items described an act in such a way that it was not clearly implied to be immoral. Participants were asked whether they agreed with the item. All items were factor analyzed. The analyses were conducted with the unweighted least squares extraction and Varimax rotation, separately for each gender and age subgroup. Results show that age differences in the evaluation of three immoral behaviors were greatest between pre-adolescence and mid-adolescence. In each age group, boys accepted all immoral acts more easily than girls. There was also a flexible use of various rationales to justify immoral acts in middle and late adolescence. 2 tables, 1 figure, and 53 references