We present the first results of U37K' and &egr;alkenone from Emiliania huxleyi grown in chemostat cultures at constant temperature but at variable growth rates. A small change in U37K' with growth rate was found for a noncalcifying strain of E. huxleyi, whereas results from a calcifying strain showed no systematic variation between U37K' and growth rate. We conclude that nutrient-limited growth rate effects will not produce serious errors in paleotemperature determinations using U37K or U37K'. However, U37K' values for the two strains differed significantly and apparent temperatures calculated using unsaturation ratios obtained from chemostat experiments are very different from those derived from batch culture experiments. Results of isotopic analyses of alkenones and bulk cells indicate no effect of growth rate on &egr;alkenone (&egr;alkenone=4.2¿0.2?). Constancy of &egr;alkenone values is a requirement for environmental and paleoenvironmental interpretations. These results indicate that isotopic analyses of alkenones are ideal markers for paleo-<CO2(aq)> determinations. ¿ 1998 American Geophysical Union