The child's understanding of open systems, as exemplified by an ecosystem and a socio-economic system, was assessed in a Piagetian type interview with 8 males and 8 females in each of grades 3, 5, 7, 9, 11 and first year post-secondary (n=96). Since Piagetian theory has been based on tasks using mainly inanimate, physical content, the generalizability of Piagetian stages and sequences to the two open systems content domains was tested. Tasks assessing the four concrete operations examined were repeated in each of the physical, the bio-ecological and the societal domains. Typical stage and sequence patterns were observed in all three domains. Post-concrete operations were represented by three formal operations in the physical domain and four systemic operations in each of the open systems domains. Logical and philosophical arguments for the qualitative difference between formal and systemic logic were presented. Three blind judges reached spontaneous agreement on 84.6% of the scores assigned for the systemic task protocols. A scalogram analysis and comparisons of the differences between pass/fail proportions indicated that the systemic operations of systems synthesis and transitive recycling were more difficult than the formal operational tasks by a Guttman step of the same size as that between the formal and concrete stages. A cluster analysis showed those most difficult systemic tasks to be grouped as if they were a part of a separate structure d'ensemble. Further analyses indicated that the greater difficulty of these two systemic operations could not be attributed to the greater un-familiarity of the task contents. Systemic task success rates were zero for respondents below grade 9 (14 years) and consistently fell far below formal task success rates for same aged peers'. The most difficult systemic operations satisfied the criteria for membership in a fifth stage as well as any other Piagetian operations do for their imputed stage membership. Nevertheless, an alternative interpretation construing systemic operations as post-concrete developments parallel and complementary to formal operations could not be ruled out. The implications of the findings for the areas of cognitive development, social development and social psychology were discussed. / Arts, Faculty of / Psychology, Department of / Graduate