This paper discusses the notion of physis in the fragments of the Pythagoreans Philolaus of Croton and Archytas of Tarentum. Building on the twentieth-century discussion of the two basic meanings of physis, ‘growth’ and ‘being’ (section 2), it argues that Philolaus was most probably the author of the first treatise entitled Περὶ φύσεως, as the first-century BC writer Diogenes of Magnesia testifies. The remaining evidence on Presocratic books entitled Περὶ φύσεως is late and unreliable (section 3). ἁ φύσις in Philolaus B 1 and 6 denotes ‘all that exists’; the Pythagorean speaks of physis in a generalized collective sense as of everything that came into being and exists in the world-order (section 4). As distinct from Philolaus, Archytas did not develop a doctrine of principles, and his epistemology was not constrained by metaphysical presuppositions. Archytas B 1 considers physis from both cosmological and epistemological points of views, as ‘the nature of the whole’ that is available to human cognition. Without setting any conditions or limitations to this process, as Philolaus did, he reinforces the latter’s declaration that “all the things that are known have number” (B 4) by making four Pythagorean mathēmata the principal cognitive tools for scientific enquiry into nature (section 5).