Parasitella parasitica is a zygomycetous mycoparasite with several hosts in Mucorales. During the infection process, it transfers nuclei and other cellular components to its hosts through biotrophic fusion. Gene transfer was proven by the formation of para-recombinants after infection with the genetic background of the host plus some additional traits from the parasite. In this thesis, natural horizontal gene transfer system has been studied for the first time at the molecular level with respect to chromosomally located genetic markers. For this purpose, a P. parasitica adenine auxotrophic mutant was employed as DNA donor, whereas a methionine auxotroph A. glauca served as recipient. The resulting methionine prototrophic hybrids were selected and analyzed for their properties: 1) Inability of the A. glauca auxotroph to convert homoserine to homocysteine in feeding experiments led us to identify the genes for homoserine O-acetyltransferase (Met2-1, Met2-2) as most probable point of mutation. These genes also identified in P. parasitica. 2) Hybridization patterns and nucleotide sequences of the A. glauca mutant revealed a large insertion in the Met2-1 gene in comparison with its wild type strain. 3) Transformation experiments with constructs containing the Met2-1 gene as selective marker could recover methionine prototrophic growth, while the Met2-2 gene could not. Beside Met2-1, GFP gene was applied as the reporter gene. 4) Hybridization of para-recombinant DNAs with Met2-1 and Met2-2 probes of A. glauca and P. parasitica showed that recombinants had acquired part of the Met2-1 gene from P. parasitica and a duplicated part of A. glauca Met2-2 gene in extrachromosomal form that enabled them to grow in absence of methionine. This way of foreign DNA establishment is a common feature in mucorales and regarded as the main reason for observed mitotic instability during subsequent sporulation cycles that discussed by possible defense mechanisms, directed against invading DNA.