The results of intra- and inter-strain crosses carried out in and between the strains from Spain, Poland, and Turkey, and the standard one (510, Scotland) of Paramecium novaurelia are presented. The inter-strain crosses were also made between the studied strains. The results showed that strains coming from the distant regions of Europe (i.e. from Spain, Poland, Scotland) and from Asiatic part of Turkey were not genetically differentiated within P. novaurelia. In the studied species of the P. aurelia complex there exists a type of breeding system which can be called moderate inbreeding.