The amplified fragment length polymorphism (AFLP) was used to study genetic variation in Pyrenophora teres causing net blotch of barley in Finland. The mean similarity was 93% between all isolates and a bit higher within two distinct populations based on 175 AFLP markers. Despite the high genetic similarity, 70 unique AFLP genotypes were identified among 72 isolates. Most of the genetic variation (68.5%) was observed within a field population and a smaller portion (30.3%) between them. Significant genetic differentiation (Fst=0.308, P<0.001) was identified between field populations. However only 1.2% of the variation was observed between mating types within a field and a lack of genetic differentiation (Fsc=0.017, P=0.087) was observed. The occurrence of the form of blotch (spot type, f. sp. maculata, or net type, P. teres f. sp. teres) was identified with specific PCR. All isolates were found to be of the net type. The existence of both mating types (MAT1 and MAT2) was identified for the first time in Finland and the ratio of the two mating types was almost 1[ratio ]1 in both locations. The evolutionary potential and the possibility of sexual reproduction of P. teres occurring in Finland are discussed.