Since the yeast Phaffia rhodozyma was first described some 35 yr ago, there has been significant interest in the development of com. processes to exploit its ability to produce carotenoids (.apprx.80% astaxanthin). However, the optimal conditions for carotenoid prodn. are not well understood. A key limitation has been the lack of an appropriate sensor for online carotenoid quantification. In this study, an in situ Raman spectroscopy probe was used to monitor intracellular carotenoid prodn. for three consecutive P. rhodozyma fed-batch expts. 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, n1 (1513 cm-1, C=C stretch), n2 (1154 cm-1, C-C stretch), and n3 (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 expt. was directly applicable to others. [on SciFinder (R)]