Coelothrix irregularis (Harv.) Børgesen, until recently a member of the Rhodymeniaceae, has had several familial assignments within the Rhodymeniales. While suggested by some that the genus belonged in the Champiaceae on the basis of morphological characters, the transfer of Coelothrix to that family has only recently been made and that effected on the basis of molecular evidence. Additional morphological characters for the species as well as DNA sequence analyses from SSU and rbcL genes further support the placement of Coelothrix in the Champiaceae rather than the Rhodymeniaceae. Coelothrix represents a sister taxon to genera in the Champiaceae: Chylocladia, Gastroclonium and Neogastroclonium. Tetrasporangial origin is the result of a single division by a cortical cell, and the tetrasporangial division pattern in Puerto Rican plants appears to be principally decussate cruciate.