Examination of type material, comparative studies on vegetative characters and data from published accounts of sporangia, gametangia and spermatozoids are used here to further develop and strengthen generic and species concepts in Zonaria and Homoeostrichus. Zonaria is characterized by octosporangia which lack a stalk cell and occur among whitish paraphyses, by the presence of cortical cells in indusiate sporangial and oogonial sori, and by slightly elongate sterile cells bordering the antheridial sori. Other characters (mucilage in sporangial and oogonial sori, biflagellate sperm with a row of many tall spines along the anterior flagellum) may also be diagnostic of Zonaria but have not been demonstrated in the generitype. Eight species are retained in Zonaria. Generic placement of Zonaria stipitata, described on vegetative characters, requires confirmation. Homoeostrichus, as typified by Homoeostrichus sinclairii, is defined by the presence of brown paraphyses in nonindusiate sporangial and oogonial sori and bordering antheridial sori, a multicellular sporangial stalk, oogonia without a stalk cell, and uniflagellate spermatozoids. Zonaria harveyana and Homoeostrichus olsenii are similar in sporangial structure, but they cannot be assigned to either Zonaria or Homoeostrichus as now defined and consequently are transferred to the new genus, Exallosorus, as Exallosorus harveyanus (Pappe ex Kützing) comb. nov. and Exallosorus olsenii (Womersley) comb. nov. Exallosorus is distinguished from Zonaria and Homoeostrichus using sporangial characters: tetrasporangia have a stalk cell and are closely packed into indusiate sori which lack paraphyses and mucilage. Problems in obtaining reliable species identification for the five southern Australian species of Zonaria are partially resolved. This study demonstrates again the essential contribution of reproductive characters to the definition of taxa and supports the position that decisions to erect, subsume or transfer members of the Dictyotales should be based on more than vegetative characters.