Abstract: Identification of cytotoxic constituent of Indonesian sponge Kaliapsis sp. has been conducted. The structure identification was judged based on the spectroscopic data, namely, ultraviolet, MS, one and two-dimensional 1H-NMR and 13C-NMR methods. The cytotoxic constituent was identified as 1-(tetrahydro-4-hydroxy-5-(hydroxymethyl)furan-2-yl)-5-methyl pyrimidine-2,4(1H,3H)-dione. This constituent hasn’t been isolated from sponges as natural product.