The phototoxicity of polyacetylenes from representatives of the Asteraceae to a broad range of freshwater and marine algal species was observed. In the tests triacetylenes were more toxic than diacetylenes. In addition, a small group of polyacetylenes with terminal alcohol or aldehyde groups were found to be toxic with or without ultraviolet (UV) treatment (300–400 nm). Quantitative tests withEuglenaandValoniaindicated toxic reactions with three compounds at concentrations greater than 1 ppm. Considering the toxicity of polyacetylenes to bacteria, fungi, invertebrates, and fish, the present work with plants is further evidence of their general cytotoxic nature.