Thecamoeba aesculea n. sp. was isolated and described from the surface of the bark of Aesculus hippocastanum and from terrestrial mosses growing on it. This amoeba is superficially similar to Thecamoeba sphaeronucleolus, but comparison of the newly isolated strain with the photographs and video records of the type strain of this species reveals differences which show that the two strains do not belong to the same morphospecies. The data obtained indicate the necessity of further comparative studies on the diversity of thecamoebian 'morphospecies' to outline clearer borders between them.