Fragments of LSU rDNA and COI mtDNA genes were sequenced in Paramecium tetraurelia strains originating from different continents, i.e. from Australia (Sydney), Europe (Spain, Poland), Asia (Israel, India, Japan) and North America (Indiana) in order to investigate intra-specific polymorphism in this species. Phylogenetic trees (based on analyses using the NJ, MP and BI methods) revealed that P. tetraurelia strains from Australia, Europe, North America and Asia (Israel, Japan) belong to one group divided into two main clusters, while a strain from India is separate and belongs to a different group. The Indian strain groups together with strains representing different species of the P. aurelia complex: P. septaurelia, P. octaurelia, and P. dodecaurelia. Polymorphism within P. tetraurelia was confirmed, however, it seems that the applied markers did not explain the ways of divergence of strains within species (Indian strain and others), and also did not show correlations between geographic origin of strains and their genetic diversity. Some species of the P. aurelia complex seem closely related.