4‐Hydroxybiphenyl and 2‐hydroxybiphenyl are major and minor metabolites, respectively, in mammals and a number of fungi. Both compounds were tested against cultures of the red tide organismPtychodiscus brevis(formerlyGymnodinium breve). The LD50value for 4‐hydroxybiphenyl is slightly lower than 2‐hydroxybiphenyl. The value for either is between 1–2 ppm under the conditions of the assay, i.e. extent of lysis within a 20‐hour period. Both substances appear to exhibit a saturation effect above 2 ppm.