The purpose of this study was the development and validation of a paper-and-pencil instrument to assess the formal stage of development as defined by Piaget. Initially, a 42-item test was constructed; item content included syllogisms, verbal analogies, combinatorial and probabilistic reasoning, and questions similar to Piagetian tasks. This instrument was administered to a group of 50 students in grades 9, 11, and 13, together with 5 individually administered Piagetian tasks. Biserial coefficients, based on performance on the 5 tasks, were used to estimate item validity. On the basis of these estimates, 24 items were selected for the final form which was administered to 78 students in the same grades, again with the 5 tasks. Biserial coefficients were computed to estimate internal consistency and item validity. Concurrent validity calculated by the Pearson r for the objective test with the tasks was .853. Reliability computer by Kuder-Richardson formula 20 was .825. The most valid items proved to be verbal analogies and certain items derived from Piagetian tasks. (Author/SD)