The effect of oxygen transfer rate (OTR) on beta-carotene production by Blakelsea trispora in shake flask culture was investigated. The results indicated that the concentration of beta-carotene (704.1 mg/l) was the highest in culture grown at maximum OTR of 20.5 mmol/(I h). In this case, the percentage of zygospores was over 50.0% of the biomass dry weight. On the other hand, OTR level higher than 20.5 mmol/(1 h) was found to be detrimental to cell growth and pigment formation. To elucidate the effect of oxidative stress on P-carotene synthesis, the accumulation of hydrogen peroxide during fermentation under different OTRs was determined. A linear response of beta-carotene synthesis to the level Of H2O2 was observed, indicating that beta-carotene synthesis is stimulated by H2O2. However, there was an optimal concentration of H2O2 (2400 mu M) in enhancing P-carotene synthesis. At a higher concentration of H2O2, P-carotene decreased significantly due to its toxicity. (c) 2005 Elsevier Inc. All rights reserved.