Phytoplasmas (genus 'Candidatus Phytoplasma') are intracellular plant pathogenic bacteria whose hosts and vectors are insects that feed on the phloem of plants. It is not possible to cultivate them in pure culture in vitro, and therefore research is difficult. They are of great economic importance and are associated with major losses in the production of many crops worldwide. They are the causative agents of grapevine yellows (GY) and seriously threaten European vineyards. They are associated with infection with phytoplasmas bois noir (ribosomal subgroup 16SrXII-A; 'Ca. P. solani') and flavescence dorée (ribosomal group 16SrV). The aim of this thesis was the multigene typing of phytoplasma isolates 'Ca. P. solani' obtained from grapevine, nettle, and elm samples from two selected locations in Istria. I confirmed the presence of phytoplasmas by using triplex real-time PCR test, followed by phylogenetic analyses secY, stamp, vmp1 and tufB gene sequences which were previously amplified by PCR. A greater diversity of genotypes was assessed for the secY and stamp genes, while vmp1 and tufB were less variable. A multigene analysis of an isolate of BN phytoplasma from elm was done for the first time. These analyses enabled a more detailed insight into the epidemiology of phytoplasma in Istria and further data on the presence of yet unrecorded collective genotypes of phytoplasma 'Ca. P. solani' in Croatia.