Prymnesium parvum is a toxin-producing harmful alga that has caused ecological and economic damage worldwide. Effective methods to control blooms of this species in the field, however, are unavailable. This study examined five natural compounds present in the invasive plant Arundo donax and one synthetic derivative (5,6-dichlorogramine) for their effect on P. parvum growth. All compounds except one inhibited growth in the following order of potency: ellipticine >  > 5,6-dichlorogramine > 1 H-indole = 2,4,6-trimethyl-benzonitrile > gramine. Ellipticine was by far the most potent inhibitor, with full algicidal activity at concentrations as low as 0.04 mg L⁻¹ and 3- and 9-day IC₅₀ values of 0.012 and 0.007 mg L⁻¹, respectively. A reduction in chlorophyll content and swimming activity and an increase in length and volume (swelling) were documented in algal cells exposed to 0.01–0.02 mg ellipticine L⁻¹. These results show that ellipticine is among the most potent natural algicides identified to date. The sixth compound tested, oleamide, unexpectedly stimulated algal growth above control levels. Overall, these observations confirm the existence of highly potent anti-P. parvum allelochemicals in giant reed and demonstrate potential for using products derived from this plant in the development of natural, environmentally friendly methods to control harmful algal blooms.