Steinernema feltiae belongs to the feltiae-kraussei-oregonensis group, clade III, and is an ubiquitarian species of entomopathogenic nematode. It is found in all types of soil and in all types of habitat. Species identification in the entomopathogenic nematodes genera Steinernema is a very complex task, given the broad variability of both morphological and biological traits within populations of a single species. To accomplish this, molecular techniques have been adopted which, however, require additional knowledge. Particularly relevant would be the possibility of testing in a reliable way the variability between different populations of the same species, which might represent different strains with different biological properties. During numerous samplings in Italy, several strains of S. feltiae were isolated. In this paper we analyze the intraspecific variability of the main morphometric and biological data of juveniles and males of 50 Italian populations of S. feltiae. The aim of our work was to determine if morphometric and biological analysis were useful to identify characters having significant diagnostic value, allowing to reliably discriminate among strains. Seven characters routinely computed for morphology (5 morphometrics for infective juveniles, spicula and gubernaculum shapes for males) and 2 biological performances (time to achieve adult stage, reproduction and progeny) were considered. The results showed extreme variability from both morphological and biological points of view