The effects of seven stable chloro-organic compounds formed during chlorination of domestic waste-water on the growth of selected fresh-water and marine phytoplankton were determined. The uptake and metabolism of selected chloro-organic chemicals by the phytoplankton were also investigated. 3-Chlorophenol, 3-chlorobenzoic acid, 4-chlororesorcinol, 5-chlorouracil, 5-chlorouridine, 6-chloroguanine or 8-chlorocaffeine at a concentration of 0.1 ppm, alone or in combinations of up to 4 chemicals, had no significant effect on the yield of Scenedesmus obliguus, Selenastrum capricornutum, Microcystis aeruginosa, Dunaliella tertiolecta, Skeltonema costatum, Thalassiosira pseudonana, and Porphyridium sp. 4-Chlororesorcinol and 5-chlorouracil were taken up by certain species but neither chemical was accumulated to a high level. The uptake of chlororesorcinol was considerably greater than that of chlorouacil. The uptake of 3-chlorobenzoic acid by the phytoplankton was negligible. 4-Chlororesorcinol was readily degraded in aqueous solution by the action of simulated sunlight and both Skeltonema and Selenastrum took up chlororesorcinol as well as its photodegradation products from the medium. Neither Skeltonema nor Selenastrum were able to metabolize 4-chlororesorcinol in the dark but appeared to transform it to some extent into more polar material(s) in the light.