A cognitive developmental theory of reading was proposed and tested in a longitudinal study that (1) compared the development of conceptual reasoning (Piaget's "operativity") and oral language (vocabulary and grammar closure) of 148 Australian children during the first 5 years of school and (2) traced the relationship of these variables to reading during that time through multitrait/multitime analysis. Results indicated that the development of operativity is surgent and discontinuous while oral language is stable and continuous; the influence of language awareness, IQ, or reading instruction could not account for these developmental characteristics. Findings suggested that the operativity measures of this study are tapping a powerful developmental influence that is partly distinct from oral language in its effects on reading. Analysis indicated that this influence was part of a broad spectrum of change in mental processing that occurs during middle childhood. Emerging conceptual capacity was found to be a necessary although not sufficient condition for the development of reading ability. As well as exerting an immediate influence, findings showed that operativity produces a pervasive delayed effect in its causal relationship with reading comprehension that, during the early school years, slowly takes on the decision-making, conceptual reasoning character of that operational influence. (JD)