Seasonal alkenone concentrations, production rates, and unsaturation patterns (U37K') were measured at station ALOHA in the oligotrophic subtropical North Pacific. Highest alkenone concentration and production rates were found in (winter and fall) or just below (summer) the surface mixed layer. Lowest alkenone concentration and production rates were found within the deep chlorophyll maximum layer (DCML). In the DCML, which occurs at 80--120 m throughout the year, U37K' temperatures overestimated water temperatures by ~2¿--4¿C. This result probably reflected the effect of light limitation on the physiology of alkenone-producing algae. At the depth of maximum alkenone production, U37K' temperatures underestimated water temperature by ~2¿--4¿C in summer and fall but overestimated in situ temperatures by ~1¿--2¿C in winter. The underestimate of measured water temperature in summer and fall most likely reflected a physiological response to limited nutrient availability. The U37K' temperature overestimate in winter was best explained by a change in the ecology of alkenone-producing algae.