Growth of Poria ambigua Bres. was improved by the addition of yeast extract and other natural materials to a basal medium of mineral salts, dextrose, casein hydrolysate, purine and pyrimidine bases, and B vitamins. Light reduced growth but was required for formation of basidiospores. The addition of natural materials largely eliminated the inhibitory action of light on growth, and with the addition of sufficient natural material growth in the light exceeded growth in the dark. On rich media, spore production was reduced or eliminated. Local inhibitions of growth on rich media permitted spore production to occur. P. ambigua is very sensitive to light. Exposure of inoculum to light for a period of minutes affected growth and spore production. The light stimulus was transmitted into mycelium formed later in the dark. Some isolates prepared from single basidiospores formed spores on nutritionally poor media. Isolates grown from single hyphal tips varied in ability to form basidiospores; one equalled in potency the stock culture used in this investigation. To explain the results, 3 growth substances are assumed as follows: (x) formed in the light required for reproduction but involved in vegetative growth also; (y) formation interfered with by light, required for vegetative growth; (z) formation favored by light, limiting for growth in the dark in the presence of luxus amounts of (y).