The effect of a number of inhibitors of oxidative and photosynthetic phosphorylation on the rate of Euglena phototaxis has been determined. Compounds which prevent oxidative phosphorylation (cyanide, azide, rotenone, 2,4-dinitrophenol, dichlorophenol indophenol) affect motility and positive topophototaxis equally. Of the compounds which prevent photophosphorylation, some (salicylaldoxime, tetramethyl thiuram disulfide) inhibit both motility and phototaxis, whereas others (ethanol, dichlorophenyl dimethylurea, methyl octanoate, carbonyl cyanide p-trifluoromethoxyphenylhydrazone) specifically inhibit positive topophototaxis. Recovery from the inhibition is found to occur after about 48 hours of exposure to the inhibitors. At least with the dichlorophenyl dimethylurea, this is probably due to a by-passing of the metabolic block. In the case of two of these compounds (dichlorophenyl dimethylurea, methyl octanoate), simultaneous exposure of the organisms to reduced dichlorophenol indophenol completely reverses the inhibition of phototaxis. Antimycin A is relatively ineffective as an inhibitor of either motility or phototaxis. However, it is found to accentuate what appears to be a manifestation of a “memory” in the phototactic response. It is concluded that the main energy source for positive topophototaxis is the photophosphorylation system of photosynthesis, that a reservoir of high-energy compounds exists, and that the primary light event in phototaxis in some way triggers the release and utilization of the material in this reservoir.