Of six strains of Mortierella tested, Mortierella alpina ATCC 32222 produced the highest yields of arachidonic acid. Supplementation of soy flour (1% w/v) and vegetable oils (1% v/v) significantly increased the biomass, lipid content and arachidonic acid level. Replacement of NaNO3 with corn steep liquor (1% w/v) also improved arachidonic acid production. A fed-batch culture system at 25 degrees C, producing a high biomass (52.4 g/l) and arachidonic acid content (9.1 g/l) in 8 days, was developed. A fed-batch system at low temperature (15 degrees C) gave even higher arachidonic acid levels (11.1 g/l) in 11 days.