Three novel bromine-containing amides, clathrynamides A (1), B (2), and C (3), were isolated from a marine sponge Clathria sp. Their structures were determined on the basis of spectroscopic data. Clathrynamide A (1) significantly inhibited cell division of fertilized starfish eggs and also showed inhibition of the growth of human myeloid K-562 cells in vitro. In the course of our search for biologically active compounds from marine organisms, we have found that the methanolic extract of a marine sponge Clathria sp. strongly inhibits cell division of starfish (Asterias pectinifera) embryos. Bioassay-guided purification of the crude extract resulted in the isolation of three novel bromine-containing amides which we have named clathrynamides A (1), B (2), and C (3). In this paper, we report the structures of 1, 2, and 3 which have been deduced from spectroscopic data.