Apple poliferation (AP) is a disease caused by 'Candidatus Phytoplasma mali' that is present and widespread in most European countries. Although infection does not lead to the degradation of plant hosts, due to a significant reduction in the size and quality of the fruits of economic losses are significant. Phytoplasma (genus 'Candidatus Phytoplasma') are intracellular, pleomorphic plant pathogens from the class Mollicutes. They are located in the core of tissues of infected plants are propagated and live only in the phloem plant and insect cells. Disease is transmitted by infected planting material and insects - vectors of the order Hemiptera. As it is still almost impossible to obtain pure culture of phytoplasma in an artificial medium, multigene characterization in often used for their characterization. 'Ca. Phytoplasma mali' has only recently been identified in Croatia for the first time by molecular methods. This thesis main objective is to characterize the newly discovered 'Ca. Phytoplasma mali' isolates from infected tissue apples and insect-vector Cacopsylla picta analysis of gene secY and imp. The presence of phytoplasma infection 'Ca. Phytoplasma mali' was demonstrated by amplification and restriction analysis of 16S rRNA.Fragments of these genes were amplified by PCR using specific primers and sequenced. Phylogenetic analysis of the nucleotide sequences obtained provided information on the variability of these genes as well as their relatedness to already described isolates. Results of the analysis of variability of additional molecular markers to the 16S rRNA are important for elucidating the molecular epidemiology, especially because this phytoplasma is transmitted by insects of the genus Cacopsylla that are present in Croatia.