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Riboflavin‐binding proteins could be the photorecep‐tors for tropism in the fungus Phycomyces blakesleeanus. Radio‐labelled riboflavin bound to both membrane‐associated and cy‐tosolic sites. The membrane sites (approximately 0.2–0.6nmol per g fresh material) were highly specific, with decreasing affinities for riboflavin (KD≅ 1 μM under reducing conditions, KD≅μM under oxidizing conditions), FMN, roseoflavin, and FAD. These binding sites, whose properties were similar to those of higher plants, could be solubilized with mild detergents, and were found in all vegetative parts of the fungus, including the spores. Mutants defective for phototropism did not differ from the wild type in the amount of binding sites or their affinity. A completely different binding to riboflavin was observed in the cytosolic supernatant of the sporangiophores; this activity was heat resistant and the binding sites could be partially purified and recognized as a polymerization product of gallic acid. Flavms were abundant in the sporangiophores (4.5 nmol per g fresh mass) and the spores (60 nmol per g fresh mass), but scarce in washed membranes (0.02–0.11 nmol per g fresh spor‐angiophore mass). Autogenous fluorescence, whose absorption and emission wavelengths fit those of riboflavin, was seen by confocal microscopy, in part as clustered particles, in the actively growing parts of the mycelium, in the cytoplasm of sporangiophores, and in the spores.

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