Representatives of all classes of Ciliophora have been studied for the detection and investigation of both prokaryotic and eukaryotic (not algal) endo- (EnS) and ectosymbionts (EcS). Different methods including transmission electron microscopy (TEM) and fluorescence in situ hybridisation (FISH) have been used. Apparently, the capability of keeping symbionts varies among the different ciliate groups as it generally is the case in different protist taxa. Most of the prokaryotic EnSs detected belong to Alphaproteobacteria. Holospora or Holospora-like infectious bacteria of this group were found in representatives of Heterotrichea, Armophorea, Phyllopharyngea, Prostomatea and mainly of Oligohymenophorea. Bacteria associated with bacteriophages were found in species of Heterotrichea and Oligohymenophorea. This holds true also for bacteria with R-bodies. A quite rare type of EnS – motile bacteria – was found in ciliates of the same two classes as well, either in the cytoplasm (Heterotrichea) or in the macronucleus and its perinuclear space (Oligohymenophorea). EcSs are more common in Heterotrichea, Armophorea and Plagiopylea, but were never found in other groups. Among the eukaryotic EnSs of ciliates, very few representatives of Microsporidia and Trypanosomatidae were recorded. In conclusion, heterotrichs and oligohymenophoreans are the most promising groups of Ciliophora for the investigation of symbiosis.