Abstract This study documented lorica ultrastructure variation in Corresponding author: and related taxa from natural populations. On that basis, inaccuracies in the taxonomic system of euglenoid Trachelomonas are pointed out. Loricae of T. caudata and taxa very similar to it were examined by light and scanning electron microscopy and by energy-dispersive X-ray spectroscopy (EDS). The latter observations showed similarity of chemical composition between loricae of different putative taxa, with silicon and iron as the main components, and the absence of a chemical element considered to be a very important component of trachelomonad envelopes - manganese. All observed morphotypes were classified in one complex based upon envelope configuration, and the “caudatae complex” was established. Our morphological analyses led us to conclude that f. pseudocaudata of T. caudata should be subsumed in the typical form or treated as a synonym of T. caudata, as the morphological differences between them are within the ty...