Since the yeast Phaffia rhodozyma was first described some 35 years ago, there has been significant interest in the development of commercial processes to exploit its ability to produce carotenoids (similar to80% astaxanthin). However, the optimal conditions for carotenoid production are not well understood. A key limitation has been the lack of an appropriate sensor for on-line carotenoid quantification. In this study, an in situ Raman spectroscopy probe was used to monitor intracellular carotenoid production for three consecutive P. rhodozyma fed-batch experiments. Raman spectroscopy is particularly well suited to the study of carotenoids due to a resonance effect, which greatly enhances the intensity of the three fundamental carotenoid bands, nu(1) (1513 cm(-1), C=C stretch), nu(2) (1154 cm(-1), C-C stretch), and nu(3) (1003 cm(-1), CH3 rock). For all three cultures, the peak height of these bands was linearly correlated with intracellular carotenoid content (1 to 45 mg/L) to a precision of better than 5%, and the correlation from one experiment was directly applicable to others. (C) 2003 Wiley Periodicals, Inc.