beta-Carotene synthesis was increased from a negligible amount to 152 mg (g-dry cells)(-1) and H2O2 was accumulated up to 16.7 mu M during 2.5 day-culture of Blakeslea trispora. When cells were cultivated in 250 ml flasks containing various volumes (25-150 ml) of the medium, not only H2O2 accumulation but also beta-carotene synthesis increased as culture volume decreased. Addition of H2O2 (10 mu M) to the 1.5-day old cultures of B. trispora resulted in 46% higher beta-carotene synthesis than that without addition. All these results indicate that beta-carotene biosynthesis is stimulated by H2O2 in B. trispora.