N-Phenylethylamides 1a-1f, were isolated from cultures of three limnic strains GW90a, GW102a and GW73a. Strain GW102a delivered additionally the compound cyclo(isoleucyldehydroalanyl) (2). The structure of these compounds were assigned by a detailed spectral analysis. Due to their potential use as herbicides, various related compounds 1a, 3, 4a and 4b were synthesized. The minimum inhibitory concentration (MIC) against Chlorella vulgaris, Chlorella sorokiniana, Chlorella salina and Scenedesmus subspicatus ranged from 100 to 12.5 microg/ml. All these amides were found to be inactive against Mucor miehei, Candida albicans, and some bacteria.