The CLAW hypothesis argues that a negative feedback mechanism involving phytoplank- ton-derived dimethylsulfoniopropionate (DMSP) could mitigate increasing sea surface temperatures that result from global warming. DMSP is converted to the climatically active dimethylsulfide (DMS), which is transferred to the atmosphere and photochemically oxidized to sulfate aerosols, leading to increases in planetary albedo and cooling of the Earth's atmosphere. A shipboard incubation experiment was con- ducted to investigate the effects of increased temperature and pCO2 on the algal community structure of the North Atlantic spring bloom and their subsequent impact on particulate and dissolved DMSP con- centrations (DMSPp and DMSPd). Under 'greenhouse' conditions (elevated pCO2; 690 ppm) and elevated temperature (ambient + 4°C), coccolithophorid and pelagophyte abundances were significantly higher than under control conditions (390 ppm CO2 and ambient temperature). This shift in phytoplankton com- munity structure also resulted in an increase in DMSPp concentrations and DMSPp:chl a ratios. There were also increases in DMSP-lyase activity and biomass-normalized DMSP-lyase activity under 'green- house' conditions. Concentrations of DMSPd decreased in the 'greenhouse' treatment relative to the con- trol. This decline is thought to be partly due to changes in the microzooplankton community structure and decreased grazing pressure under 'greenhouse' conditions. The increases in DMSPp in the high tem- perature and greenhouse treatments support the CLAW hypothesis; the declines in DMSPd do not.