Anaplasma phagocytophilum is an emerging tick-borne pathogen. Great genetic diversity of A. phagocytophilum has been described in animals and ticks. The present study is focused on the genetic variability of the groESL operon of A. phagocytophilum in human patients in Slovenia. During 1996–2008, there were 66 serologically confirmed patients with human granulocytic anaplasmosis. Of these, 46 were tested with a screening PCR for a small part of the 16S rRNA gene of A. phagocytophilum and 28 (60.9%) were positive. Positive samples were additionally tested with a PCR targeting the groESL operon and a larger fragment of the 16S rRNA gene. All amplicons were further sequenced and analyzed. The homology search and the alignment of the groESL sequences showed only one genetic variant. Sequence analysis of the 16S rRNA gene revealed 100% identity among amplicons. Slovenia is a small country with diverse climate, vegetation, and animal representatives. In previous studies in deer, dogs, and ticks, great diversity of the groESL operon was found. In contrast, in wild boar and in human patients from this study, only one genetic variant was detected. The results suggest that only one genetic variant might be pathogenic for humans or is competent enough to replicate in humans. To support this theory, other genetic markers and further studies need to be performed.