WE report here the first in situ bioassay of short term effects of polychlorinated biphenyl compounds (PCBs) on natural marine phytoplankton communities. (Synthesis and utilization of these industrial organochlorines have been discussed before1–5.) PCB residues have been identified in water, biota and sediment samples throughout the marine environment5–9. Near Escambia Bay, Florida, industrial outfall water and sediment in the immediate outfall vicinity contained PCB residues as high as 275 p.p.b. (1 billion =109) and 486 p.p.m. respectively. Live oysters approximately 10 miles downriver of the outfall contained residues as high as 3 p.p.m.10. Seven of ten municipal wastewater outfalls sampled along the California coastline were confirmed to carry PCB compounds11. Information about the physiological and ecological effects of PCBs in natural aquatic communities is limited.