a1_The infraspecific taxonomy of the European populations of the Large Blue (Maculinea arion) is confusing. Several subspecies have been described mostly based on external morphological features. In the Carpathian Basin two subspecies have been distinguished. Maculinea arion arion flies from mid-May to mid-June and Maculinea arion ligurica is on the wing from the end of June to mid-August. The two forms show some differentiation in habitat use, but occasionally can also share habitats with two peaks in the appearance of butterflies. Our aim was to study the level and structure of genetic variation in a set of populations of the two phenologically different M. arion. Imagos were collected from 8 localities between 2000 and 2006. Enzyme polymorphism was analysed at 13 enzyme loci using polyacrylamide gel electrophoresis. In the analysis of the data, we estimated the parameters of polymorphism. To study the pattern of genetic differentiation F-statistics, hierarchical F-statistics and AMOVA were computed. GeneClass and Structure were both applied to analyse the differentiation between the two phenologically different sets of populations. Cavalli-Sforza and Edwards' arc distances were calculated and a UPGMA dendrogram was constructed on the basis of the distance matrix. PCA analysis was also carried out using the allele frequencies of the individuals. The level of polymorphism was relatively high in M. arion. The results of all analyses indicated that the differences between the two sets of phenologically different populations accounted for a low percentage of the total differentiation. In addition, a sizeable amount of variation could be attributed to the differences among the samples collected from the same population in consecutive years. Thus, we concluded that the "spring" and "summer arion" could not be considered as separate ESUs, although we could attribute conservation value to both forms on the basis of, a2_their phenological differentiation and habitat use., and Judit Bereczki, János P. Tóth, Andrea Tóth, Edit Bátori, Katalin Pecsenye, Zoltán Varga.