Phenalamides $A_1{sim}A_3$ were reisolated as cytotoxic substances from culture broth of Myxococcus stipitatus JW111. The producing strain was isolated from the marine sediment collected off the shore of Geomun Island, Korea. The active principles were extracted from cell mass with acetone and successively purified by silica gel column chromatography, Sephadex LH-20 column chromatography, and finally recycling prep. HPLC. These compounds demonstrated significant cytotoxicity against certain human cancer cells, having $IC_50$ values ranging from 0.23 to 0.50 ${mu}g/ml$. Moreover, they also inhibited the growth of adriamycin-resistant HCT/ADM human cancer cell line as well as its parent sensitive cell line.