Parasitella parasitica (zygomycetes, Mucoraceae) is a fusion parasite of many but not all Mucor-like fungi. During the fusion process with the hosts, genes are transferred from the parasite to the hosts. This parasexual system has been analysed with respect to complementation of auxotrophic mutations in the host(Absidia glauca)with genes from Parasitella parasitica, transferred during infection. This system is a very efficient model for studying naturally occurring horizontal gene transfer in eukaryotes. The genomic sequence of this organism provides a prerequisite for studying spreading of Parasitella genes within the host range.