Abstract
Ceramium cimbricum, originally described from Limfjorden, Denmark was isolated into unialgal culture form several localities in southern Norway. The distinguishing features include straight, often unequal apices, 4–5 periaxial cells and narrow cortical bands. Reproductive structures are described for the first time in European material and a Polysiphonia‐type of life history was demonstrated, though irregularities were also seen. Culture observations of phenotypic variability and examination of type material suggest that C. cimbricum and the widely recorded C. fastigiatum may be conspecific. However, the latter binomial is illegitimate.