Stentor polymorphus (Müller) Ehrenberg (Heterotrichea, Ciliophora) is a trumpet-form ciliate with endosymbiotic green algae (photobionts). Previous reports have indicated that this photobiont has a form typical of Chlorella Beijerinck (Trebouxiophyceae), with one chloroplast and a distinct pyrenoid. We collected S. polymorphus from an artificial pond on the shore of Lake Biwa in Japan. Microscopic examination demonstrated that there was no pyrenoid in the photobiont, and thus we analyzed the cytological, genetic, and biochemical characteristics of these algal cells. Cultured photobionts were smaller than those in hospite. Phylogenetic analyses placed the photobiont within the genus Mychonastes Simpson and Van Valkenburg, a chlorophycean alga occurring in fresh/brackish waters that lacks pyrenoids. Mychonastes provided little photosynthate to its host; the quantity of carbohydrate amounted to only 2% of that provided to Paramecium bursaria (Ehrenberg) Focker by its photobiont.