Physiological parameters such as cytoplasmic streaming and motility are used to analyze the effects of chemically different phenolic compounds. Test organisms included: Cyclotella cryptica, Dunaliella salina, Chlamydomonas reinhardii strain 137, Lemna minor and Euglena graclis. It was found that phytotoxicity of phenols, meta-isomers, and methylated phenols is lower than that of ortho- and para-diphenols. Comparing ortho- and para-isomers of the same substance, the results were not uniform, as there were organisms for which para-isomers are more toxic, and others, for which ortho-isomers were more toxic. These results suggest that most compounds tested affect unspecific cell proteins, structural proteins in cell organelles as well as cytoplasmic proteins involved in cell motility and cytoplasmic streaming. (JMT)