The hexamitid flagellate Spironucleus vortens, previously reported from Pterophyllum scalare from Florida, was found in the intestine of Leuciscus idus in Norway. The flagellate was cultivated and studied by scanning and transmission electron microscopy. Identification was based on a suite of ultrastructural features unique for S. vortens: compound lateral ridges, a swirled posterior end, and a distinctive microtubular cytoskeleton. Microfibrillar structures with a periodicity of 0.13 μm in the right peripheral part of the compound lateral ridges were shown to be responsible for the distinctive rope-like appearance of the peripheral ridge seen in scanning electron micrographs, and not previously reported for S. vortens. The present results show a wide geographic distribution and a wide temperature tolerance for S. vortens. The flagellate was successfully cultivated at 5 °C and 15 °C, having previously been cultivated between 2–34 °C. Spironucleus vortens is believed to be endemic in Norwegian waters, but an introduction hypothesis is also discussed. The similarity is striking between S. vortens and S. elegans, previously described from amphibians and fish in Europe, and the possibility of conspecificity is believed to be high.