Emiliania huxleyi is widely regarded as the most likely source of C 37C 39 alkenones in present-day seawater and Recent sediments, but other sources are required to account for the presence of alkenones in sediments that predate the first occurrence of E. huxleyi about 265,000 years ago. Analysis of the lipids of a laboratory culture of the closely related marine coccolithophorid Gephyrocapsa oceanica (strain JB02) isolated from a massive bloom in Jervis Bay, eastern Australia showed that this species also synthesizes C 37C 39 alkenones and esters of di- and tri-unsaturated C 36 fatty acids. This confirms earlier predictions based on the fossil record that species of Gephyrocapsa should contain these distinctive biomarkers. In this strain of G. oceanica the sum of the C 38 ethyl ketone concentrations is similar to, or greater than, that of the C 37 methyl ketones over the temperature range 11-29°C, whereas the reverse is true for Emiliania huxleyi. It should thus be possible to determine whether there is a contribution from Gephyrocapsa to the alkenones in seawater and sediments. The concentrations per cell of the major alkenones showed different responses to increasing growth temperature. The 37:3 and 38:3 methyl alkenones and 38:3 ethyl alkenone showed an approximately linear decrease in cellular concentration over the entire temperature range, whereas the 37:2 and 38:2 methyl alkenones and 38:2 ethyl alkenone concentrations showed almost no change from 11 to 20°C followed by a dramatic increase above 20°C. As a result, the ratio of di- to tri-unsaturated methyl alkenones as measured by U37k' changes greatly with growth temperature, but the is different from that found for E. huxleyi in culture. The temperature response can be approximated by the linear relationship U37k' = 0.049 T - 0.520 ( r2 = 0.89), although a better fit can be obtained using polynomial expressions. These data might account for some of the apparent anomalies in predictions of sea surface temperature (SST) derived from Emiliania-based alkenone-SST calibrations in those sediments which contain contributions from Gephyrocapsa.