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SUMMARY: An investigation into the effect of light on the growth of Phytophthora infestans revealed that whereas exposure to light caused diminished growth on pea medium, growth on several other media was not decreased. This diminished growth was an indirect effect of light caused by an alteration of the medium. All the P. infestans tested reacted similarly, but most other fungi examined were unaffected. Germination of zoospores of P. infestans appeared to be especially sensitive. This inhibitory effect was not removed by heat and peroxides were not, therefore, considered as the immediate cause of the inhibition. Nevertheless, following exposure to light, peroxide formation and the appearance of the inhibitory effect were closely associated. Addition of several reducing compounds to pea medium overcame the effect of exposure to light on zoospore germination. Mycelial growth was also improved by similar additions to a defined medium. This was particularly marked following the addition of glutathione (10−3 M) and L-ascorbic acid (1·0 g./l). The decrease in reducing potential during illumination might, in the light of experiments conducted with defined media, explain the observed effects of illumination.