Spent mycelia of Blakeslea trispora recovered from a previous fermentation enhanced carotene production by mated cultures of the same organism. Peak yields of 107 to 142 mg of carotene per 100 ml of medium were achieved in 6 days. beta-Carotene constituted 92% of the total carotenoids produced. The enhancing substance was found present in an aqueous extract of the mycelium. Part of its activity was due to an organic acid fraction. Other microbial cells, molds, yeasts, and bacteria were also capable of enhancing carotene production.