This paper provides information on the morphology, reproduction and distribution of a freshwater brown alga, Porterinema fluviatile (Porter) Waern (Phaeophyceae). The alga was recorded in oligotrophic water from Mlava Spring, a karst limnocrene in eastern Serbia (44°11′298″N, 21°47′021″E). This is the first record of freshwater brown algae in general in Serbia, and the first record of P. fluviatile in southeastern Europe. The thallus of P. fluviatile formed small, dark-brown tufts, with multicellular, uniseriate filaments that were irregularly branched. An anastomosis was noticeable between some filaments. The vegetative cells of the young filaments were cylindrical (17-38 µm long × 2.5-4.0 µm wide). The cells had 1 or 2, rarely 3, plate-like parietal plastids. The reproductive organs of P. fluviatile (asexual reproduction) originate from the transformation of vegetative cells. Elongated vegetative cells develop inflated vegetative cells (5-10 µm in diameter) with lipid droplets. Further transformation of these cells leads to the formation of intercalary plurilocular sporangia with 4 to 8 (rarely 16) cells. Nonmotile, round aplanospores, which develop by protoplast division in elongated vegetative cells, are described for the first time in this species.