Septoria tritici blotch (STB) caused by Mycosphaerella graminicola is a serious disease in some wheat-growing regions of Iran. Continuous monitoring of the fungus populations is indispensable to study the efficiency of known resistant cultivars and Stb genes. In this study, resistance of 49 wheat genotypes, including 20 wheat differential cultivars worldwide and 29 other wheat genotypes, were studied to 19 M. graminicola isolates. Among 931 genotype-isolate interactions, 224 isolate-specific resistances (26%) were observed by comparing the means of disease severity data using LSD 1% and LSD 5% . Iranian M. graminicola isolates showed new virulence patterns. All isolates were virulent to Stb2, Stb5, Stb6, Stb7, Stb13 and Stb14. Among wheat differentials, M3 was resistant to all isolates, indicating that it has gene(s) with the most efficiency against Iranian isolates. Among other genotypes, two genotypes were resistant to all isolates. This suggests that these genotypes have efficient resistance gene(s) to Iranian M. graminicola isolates and could be used as resistance sources to STB in Iran. They may be effective against other populations of M. graminicola worldwide.