Mycorrhizal inoculum of Glomus faciculatus, as measured by root infection and sporulation on sudan grass, increased with increasing light intensity. Compared to natural glasshouse daylight, spore numbers of G. fasciculatus also increased dramatically when mycorrhizal sudan grass was exposed to extended photoperiods with mercury vapour or metal halide lamps of high intensity. Results from both light experiments indicate that the level of sugars, but not amino acids, in root exudates from 2-month-old sudan grass plants was correlated with the spore production of G. fasciculatus. It appears that growing sudan grass at greater light intensities and under extended photoperiods will increase the quantity and quality of commercial G. fasciculatus inoculum.