Carpogonia, spermatangia and carposporangia are demonstrated for North Atlantic plants of Audouinella alariae (Jonsson) Woelkerling for the first time. The plants are monoecious. Carpogonia are terminal on short branches and give rise to short trichogynes laterally. Spermatangia are usually borne in pairs on the supporting cells of carpogonia. Fertilized carpogonia give rise to 3-4 carposporangia. Morphology of sexual reproductive structures and postfertilization development provide characteristics for distinguishing A. alariae from A. rhipidandra (Rosenvinge) Dixon, which were previously synonymized.