Fifty-six Dutch school children (aged 6-0 to 14-4 years) participated in a study designed to investigate their ability to reason with logical implication. They answered a total of 32 reasoning problems. Before or after each question they were asked a class inclusion question. A high degree of correspondence was found between class inclusion and reasoning with logical implication premises. The child's understanding of the subordinate and superordinate characteristics of the premises appeared to be a decisive factor in judging the truth of logical arguments, although many correct judgments were based on incorrect understanding of the premises. It was concluded that when children fail to solve a problem, they do not necessarily do so because they lack the particular logical operations, but because of a misunderstanding of the premises. Incomplete analysis of the problem appeared to be a second source of error. Further, understanding of logical premises was found to be improving with age. (Author)