Althoug h the bioche mical funct ions and biosynthet ic pathw ays of alkenones are still largely unknow n, alkeno ne unsat uration rati os are now used extensivel y to infer ancien t sea surfa ce tem peratu re, and thei r isotopic compo sitions have been used to recons truct ancien t atm ospheric CO 2 levels. The infer red relations hips betw een alkeno ne unsaturati on ratios, isotopic compo sitions , and grow th condit ions are based on empirica l labor atory and field studies and, in the case of isotope fract ionation, on simple models of carbon acquis ition and fixat ion. Significan t uncertaint y still exists concerning the physi ological and ecological facto rs affecting cellular production of alkeno nes, unsat uration ratios, and isotopic compo sition. Phytopl ankton cult ure condit ions have been shown to affect alkenone unsat uration (U 37 K H), cellular alkenone content, intracellular isotopic compositions (Ád), and changes in fractionation (e P) as a function of the quotient of algal growth rate and aqueous carbon dioxide concentration (m/CO 2). Such studies imply that plant physiology can affect the interpretation of environmental signals. The factor(s) controlling cellular alkenone concentrations and unsaturation ratios are reviewed, as well as the carbon isotopic composition of the alkenone-producing algae. A new technique is presented to determine growth rates of the alkenone-containing algae in natural settings that will facilitate testing laboratory-based hypotheses concerning the carbon isotopic fractionation and its relationship to growth rate/growth status of alkenone-producing algae in the field.