The relative effects of chronological age, mental age, IQ or Piagetian task performance were investigated among 143 Ss of high, average, and low IQ. Two kinds of group comparisons were made on fifteen tasks: (1) groups of the same chronological age, but different mental age and IQ, and (2) groups of the same mental age, but different chronological age and IQ. Results indicated that children of higher IQ develop through Piagetian stages faster than children of lower IQ, and that high IQ children tend to think in a more preoperational way than older children of the same mental age but lower IQ. Implications of the study suggest that it is dangerous to limit the assessment of intellectual development to traditional psychometric methods; Piagetian methods offer an important source of very different information about an individual's intellectual development. (SBT)