Bostrychia bispora sp. nov. (Rhodophyta) was collected from two localities near Darwin, Australia on Avicennia marina (Forsskål) F.K.M. Vierhapper pneumatophores on 4 June 1989. It is similar in morphology to Bostrychia moritziana (Sonder ex Kützing) J. Agardh, except that it produces bisporangia instead of tetrasporangia. Sexual stages are absent in the field and in culture. When 21–35 days old, sporelings 2–5 mm in length produce bisporangiate stichidia. Six successive bisporangiate generations have developed in culture. DAPI-fluorescence microscopy data indicate that successive generations of B. bispora have identical DNA levels that are 30–40% higher than DNA levels in B. moritziana tetrasporophytes. These data suggest that B. bispora is an apomeiotic aneuploid or triploid derived from B. moritziana. Furthermore, the alloparasite Dawsoniocolax bostrychiae (Joly et Ya-maguishi-Tomita) Joly et Yamaguishi-Tomita does not infect B. bispora, whereas it infects all isolates of B. moritziana tested. Wound healing in monosiphonous axes occurs by cell division, cell fusion and rhizoid formation. Occasionally, secondary pit connections form between axial cells of the monosiphonous germlings, a phenomenon not reported in other Ceramiales. All this evidence strongly indicates that B. bispora is a non-sexual species that is isolated genetically and evolutionarily from all other Bostrychia species.