This problematical form is a straight to slightly curved tubular micro-organism that has a conspicuous dark central canal and a distinctive radial-hyaline wall structure. The wall is usually single-layered, but multilayered walled forms are also known. The micro-organism has previously been described as Nuia siberica Maslov, 1954, from Lower Ordovician rocks of Siberia. Subsequent Russian writers have reported Nuia from additional Lower Ordovician localities in Siberia and Kazakhstan. They consider Nuia to be an alga, although they do not agree on the major algal group to which it should be assigned. The present writers review the arguments and conclude that it should be considered as a problematical micro-organism. The peculiarities of the internal structures do not permit its assignment to one of the established groups of fossil algae.