Mature thalli ofBoldia erythrosiphongen. et sp. nov. from Big Walker Creek, Giles Co., Va., are hollow, simple, or, rarely, branched tubular cylinders. The plants generally are from 1 to 20 cm in length and 0.1–1.5 cm in diameter, brownish‐red or olivaceous in color, and grow singly or in clusters from a multicellular holdfast. The vegetative portion of the thallus is essentially monostromatic, comprised of isodiametric, or slightly elongate, uninucleate cells with numerous parietal, simple or lobed, ribbon‐like chromatophore segments. Monospores are produced from narrow, branching filaments (derived from vegetative cells) which form a reticulate pattern in the intercellular matrix external to and at the juncture between groups of (usually 2, 4 or 8) vegetative cells. The spores are liberated singly and germinate into short, branched filaments. New plants arise from small, monostromatic discs of branching filaments united in a common matrix which become cushion‐like and ultimately produce saccate thalli. WhileBoldiashows affinities with certain members of the Bangiales, it cannot be assigned to any of the presently recognized families. Accordingly, it is placed in a new and distinct family,Boldiaceae,distinguished by the unique manner in which asexual spores are produced from filaments derived from vegetative cells, the filaments developing within the intercellular matrix of the thallus, as well as by the parietal chromatophores of the vegetative cells.