Nitella flexilis, the smooth stonewort, is a freshwater species of characean algae that is used as a model organism for its large cell size and relative ease of cultivation in the laboratory.

 A robust species growing up to a meter long with axes up to 1mm wide. Branches in whorls once or twice divided.

Distribution: The species occurs on all continents of the world except Australia. It has been recorded from several counties in Ireland. From the Eglinton Canal in Galway; and in Counties Down and Londonderry; River Dorree in Clare Island Co. Mayo 2004.