A new alkaloid (1), together with five known compounds (2-6), has been isolated from the stem of Sparganium stoloniferum Buch.-Ham. The structure of the new compound was elucidated as 3-isobutyl-tetrahydro-imidazo[1,2-a]pyridine-2,5-dione on the basis of physical and chemical evidence and spectral analysis. Compound 6 was obtained for the first time from the Sparganium genus.