Ulonema rhizophorum, a brown alga, is an endophyte, growing as small circular dark spots on the fronds of the introduced red alga Dumontia cordata, native to the northeast Atlantic. The spots are patches of branched filaments 1 or 'a few' millimeters wide, and 65-80 µm tall, growing from rhizoids embedded in the cortical cells of the host alga. This endophyte is native from Norway to the British Isles. 'Ulonema rhizophorum' reported from Ulva spp. in the Mediterranean, in Turkey, may be a different species. We have not found information on impacts of this endophyte on Dumontia cordata, Ulonema rhizophorum, was first collected on the East Coast in Casco Bay, Maine, in 1915 and has been found from Nova Scotia to Narraganset Bay, covering most of the range of tis host.