The relationship between total length and four other dimensions was investigated for seven southeastern Australian species of Ephemeroptera belonging to the families Siphlonuridae, Ameletopsidae Oniscigastridae and Coloburiscidae. Of the twenty‐eight relationships investigated, only nine cases of isometry were found. In most cases, there was a change in the allometric relationship late in nymphal life. However, within a given species, this change did not occur at the same total body length for all of the dimensions investigated, indicating that it was not associated with nymphs passing from an immature to a mature stage as suggested by Clifford (1970).