My main aim is to identify and discuss parallels between the epistemologies of Gottlob Frege and Jean Piaget, Although their work has attracted massive attention individually, parallels in their work have gone unnoticed. My discussion is in four parts and covers psychologism and epistemology; five epistemological criteria in Frege's rational epistemology under an AEIOU mnemonic, namely autonomy, entailment, intersubjectivity, objectivity and universality; the elaboration of these same criteria in Piaget's developmental epistemology; their implications for developmental psychology and epistemology. One main conclusion is that the same criteria fit both Frege's and Piaget's epistemology, A second conclusion is that Piaget's developmental epistemology can be regarded as an elaboration of Frege's rational epistemology in each of these five respects on both methodological and substantive grounds, Both conclusions are compatible with non-psychologism, which was accepted by both Frege and Piaget. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.