The noxious, submersed aquatic plantHydrilla verticillatahas not thrived in certain lakes, including Lake Starvation, northwest Hillsborough County, Florida. Aqueous extracts of sediment from that lake were inhibiting the growth of hydrilla in the laboratory (Co, organic carbon,ca. 0.4 ppm). In contrast, similar concentrations of active fractions did not inhibit the growth of the freshwater algaS.obliquus, though at higher concentrations, cultures were stimulated (6–12 ppm, Co), and inhibition was noted for the highest concentration studied (18 ppm Co, 0.01 ppm Fe).