On August 11, 1965, during a search for aquatic flowering plants, a large liverwort subsequently identified as Porella pinnata L. was collected near the mouth of Mud Creek at the west side of Black Lake in Cheboygan County. Specimens were found in large, luxuriant, olive-green or blackish clumps on tree roots submerged by as much as two feet or murky water. Visual evaluation of the habit of the plant was not possible owing to the darkness of the water, although it appeared to occur in proximity to, and often mixed with, Riccia fluitans L. Fragments of Porella were seen floating near the surface of the water, and depauperate clumps were observed on tree roots above the surface. The creek is cold, slow-moving, and probably slightly alkaline. The site was visited two days later and additional material collected by Miller and Crum. The only previous authenticated collections of Porella pinnata from Michigan are those made by Nichols (1935, 1938) in Marquette County in the Upper Peninsula and Gilbert (1958) in Iosco County in the Lower Peninsula. The present find confirms the reported occurence of this species in the UMBS region (Gilbert, 1958). Both of Nichols’ specimens were found attached to submerged rocks, while Gilbert listed "submerged rocks and other materials" as the substrate. An investigation of data accompanying specimens from North America in the U of Michigan herbarium confirms the fact that in northerly locations Porella pinnata is almost always found submerged, while on more southerly sites it frequently thrives on moist soil, and sometimes in rich woods. Schuster (1953) noted that in the South it occurs on cypress knees, on the bark of beeches, or as a zerophyte on rocks far from any source of constant water. He suggested the existence of a diverse series of genotypes to account for this apparent broad range of ecological tolerance, to which Conrad has also recently called attention (1968). Because of its preference for submerged substrates in the northerly reaches of its range, Porella pinnata may be more common in this area than heretofore suspected. A more thorough knowledge of the ecology of its preferred habitat may indicate that this liverwort is not rare, but mrerely local in occurrence and difficult to find. Plants of this species are characterized by leaves scarcely touching, narrow non-decurrent underleaves, and similarly tongue-shaped underlobes. According to distribution records given by Frye and Clark (1946), Porella pinnata L. is widespread in the South and East. I have examined specimens from Vt., Mass., R.I., N.J., Pa., Va., W. Va., N.C., S.C., Ga., Fla., Tenn., La., Tex., Mo., Ark., Ind., Ill., Ohio, Mich., Ont., N.S.