We examined the effects of citric acid (CA) supplementation on astaxanthin production by wild-type Phaffia rhodozyma TISTR 5730, grown in coconut water at 25oC, to reduce energy costs for tropical regions. In shaken flask cultures with 3 levels of supplement (0.05%, 0.10%, and 0.15% (w/v)), 0.15% citric acid effected the best growth (13.64 g/L) and highest astaxanthin concentration (1052 µg/g dry cell at 84 h)—3-fold higher than without supplementation (315 g/g dry cell at 84 h). Grown in a 2-L fermentor at aeration rates of 1, 2, and 3 vvm (540 rpm, 500 lux, 25oC, initial pH 5.5), however, the astaxanthin yields were 3880, 4593, and 753 µg/g dry cell, respectively. At the optimal aeration rate (2 vvm), the astaxanthin yield was approximately 15- fold higher than that of shaken flask cultures that lacked citric acid. Over-aeration (3 vvm), however, caused a severe decline in astaxanthin yield (753 µg/g dry cell at 84 h).