Amoebae of the genus Thecamoeba (Amoebozoa, Discosea, Thecamoebida) are among the most widely distributed and best-known species of naked lobose amoebae. However, the number of available ultrastructural studies of these organisms remains low. The most of them are dated back to 1960th-1970th and virtually no modern data appeared in the literature, except for the newly described Thecamoeba aesculea. As a result, we have no complete reports on the fine structure of many widely distributed and most trivial Thecamoeba species. The present study provides the complete ultrastructure of Thecamoeba quadrilineata strain CCAP 1583/10. The studied strain is also known to serve as a host for Nucleophaga amoebae – an intranuclear parasite belonging to the Opisthosporidia clade, so the investigation of non-infected amoebae provides “reference” data for studying the infection process and intracellular development of this parasite.