Net blotch caused by Pyrenophora teres Drechsler (anamorph Drechslera teres [Sacc.] Shoem) is a major foliar disease of barley ( Hordeum vulgare ) worldwide and causes economic loss. The fungus, exists in two forms, designated P. teres f. teres and P. teres f. maculata , which induce net form net blotch (NFNB) and spot form net blotch (SFNB), respectively. However, no clear morphological or life cycle differences between the two forms of P. teres have been identified, these forms could be differentiated based on their genetics and symptom on host leaves. In this study, 160 isolates were provided from barley and uncultivated grass such as Avena sativa and Hordeum murinum collected from Izeh (Khuzestan), Farasfaj (Hamadan), Shabestar (Azarbaijan sharghi), Jafarabad (Ardabil) and Bandar Torkaman (Golestan). Morphological features of all isolates were investigated. In order to confirm the morphological identification, sequences of GPDH and ITS regions were amplified for 9 and 21 isolates, respectively and they were used in phylogenetic study. Based on result of morphological examination and phylogenetic analysis, we conclude that the isolates from barley and Hordeum murinum were Pyrenophora teres and one isolate from A. sativa was P. lolii . Host studies have shown the genetics of resistance to the two forms of P. teres to be independent. So, it is necessary to identify the form of species for each isolate and study each form independently. For identification the form of species, form/mating type-specific Primers were used. Based on different size of fragment amplified by these primers, form of the isolates can be determined. Result from using these primers show that isolates collected from Izeh, Farasfaj, Shabestar and Bandar Torkaman were identified as PTM while most of isolates from Jafarabad were identified as PTT. In order to confirm the form designations by these primer sets, some isolates were inoculated on barley leaves. The symptoms examination on inoculated leaves and also sequence of MAT locus confirm the result from using mentioned primer set. Cross inoculation of P. teres isolates from all hosts, uncultivated grasses and cultivated barley, showed that uncultivated grasses could be as an inoculum sources for cultivated barley epidemics and also contribute in pathogen evolution. Also, in this study, in order to investigate the genetic structure of P. teres populations in Iran, we amplified and analyzed 7 microsatellite loci in152 isolates collected from five provinces. Analysis of the SSR data confirm the result of form/mating type-specific primers. Cluster analysis using the SSR data and UPGMA procedure revealed that all tested isolates clustered in two distinct (PTT and PTM) groups. AMOVA also revealed highly significant differences among the two forms of P. teres (PhiPT = 0.693, P = 0.001), PTT and PTM, distributing 65% of the total genetic variation. Based on these result, hybridization is rare or absent under field conditions in Iran. In order to investigate the genetic structure of PTM populations, we remove Jafarabad population which most of isolates were PTT and analyzed the SSR data of 116 PTM collected from 4 provinces. Significant genetic differentiation (PhiPT = 0.112, P = 0.001) was found among four PTM populatio 89 % of genetic variation occurred within populations and 11 % between populations. Cluster analysis conducted in Tess indicated the presence four genetic groups. We find correlation with geographic origin in the cluster analysis of 116 PTM isolates collected from four provinces of Iran. Principal coordinate analysis (PcoA) showed results similar to those obtained with Tess and separate four geographical populations by horizontal and vertical axes. Based on principal coordinate analysis, Izeh and Hamadan showed the least genetic distance and Bandar Torkaman population was clearly separated from the other populations. Analyses of multilocus association, genotype diversity and mating type frequency suggest that Farasfaj and Izeh populations reproduce predominantly sexually while Bandar Torkaman and Shabestar reproduce asexually. Overall our study provides evidence for high genetic variation in Iranian PTM populations and high genetic variation may be the substrate for rapid adaptive evolution in this pathosystem highlighting the need for integrated efforts to control the disease. Key Words SSR, P. teres, population structure, evolution, mating types