Summary. The microscopic size of nematodes, their morphological similarity, limited number of distinguishable taxonomic characters and overlapping morphometric measurements, make them one of the most difficult organisms to identify. Mainly, various morphological and morphometrical features are used for their primary identification and characterization. Knowledge of interspecific and intraspecific variations of these features in different populations of different origins is essential and often crucial when dealing with species that are very similar to each other. Xiphinema montenegrinum from Montenegro and X. hyrcaniense from Iran represent two geographically distant but morphologically close species that can be separated by ontogeny of the tail shape and some morphometric indices. Females and males of X. montenegrinum have a slightly shorter tail, a shorter hyaline portion of the tail, and greater tail tip variation compared to females and males of X. hyrcaniense. The distance between the cloacal opening and the adanal pair of supplements in males is a good diagnostic character for separating these two species since there is no overlap in values.