Oscillatoria perornata, a cyanobacterium (blue-green alga), common in catfish production ponds in the southeastern United States, produces the monoterpene 2-methylisoborneol (MIB), which is absorbed into catfish flesh and imparts a "musty" taste, rendering them unpalatable and unmarketable. Algicides that are currently in the commercial market to control O. perornata have broad-spectrum toxicity toward other beneficial phytoplankton, such as the green alga Selenastrum capricornutum, as well as low biodegradability. As part of our continuing efforts to search for natural-product-based algicides, the ethyl acetate extract of the roots of Swinglea glutinosa was investigated. This report describes isolation and structure elucidation of one novel coumarin, two known coumarins, and nine acridone alkaloids from S. glutinosa root extracts and the evaluation of these compounds for algicidal activity against O. perornata.