Growth of Oligoneuriella rhenana (Imhoff, 1852) nymphs (Ephemeroptera: Oligoneuriidae) were studied in 2 rivers in Piemonte, northwestern Italy. The study was performed during the period of occurrence of the preimaginal stages, and 17 samplings were carried out from Apr. to July 2004. A significant difference in the size increase between the populations inhabiting the 2 streams was detected. We agree with the opinion that dissimilar thermal regimes of the 2 rivers may be the most important factor giving rise to this difference in growth.