This paper describes a fossil record from northeastern Poland of Sphagnum obtusum Warnst., a plant that has rarely been found in Holocene sediments. This species occurred circa 1710–1570 cal. BP on Mechacz Wielki peatland and was present in the transitional phase between a fen that developed on lake sediments and a bog. Sphagnum obtusum was a dominant species and was accompanied by such vascular plants such as Scheuchzeria palustris, Carex rostrata, Comarum palustre and Oxycoccus palustris. Using testate amoebae, we reconstructed the past habitat (water table and pH), with S. obtusum occurring in a moderately wet habitat (mean depth to the water table or DWT  =  9.5, SD  =  0.5) and under slightly acid conditions (mean pH  =  5, SD  =  0.1). A clear correlation was found between the distributions of the subfossil S. obtusum and those of the indicator testate amoebae Hyalosphenia papilio and Cyclopyxis arcelloides. The disappearance of S. obtusum is assumed to have been caused by a trophic shift and succession of a more acidophilic species, Sphagnum angustifolium.