SummarySeveral lines of evidence confirm that glucose moves from alga to fungus in the thallus of Peltigera polydactyla during photosynthesis. When thallus discs are incubated in NaH14CO3 solutions in the light, [14C]glucose is detectable after i minute but [14C]mannitol—the principal product of photosynthesis—is not apparent until after 2 minutes. If high concentrations (2% w/v) of [12C]glucose are included in the media, then [14C]glucose diffuses out of the tissues, no [14C]mannitol is formed, and no translocation of 14C from algal layer to medulla occurs. These effects are explained on the basis that the [14C]glucose released by the algal cells is prevented from entering the fungus because of competitive inhibition by the non‐radioactive glucose in the medium. The concentration of glucose in the medium required to achieve a 50% inhibition was approximately 0.004 M. It was found that 2‐deoxy‐glucose and 3‐methylglucose had similar effects to glucose, but fructose, galactose, mannose and mannitol had no effect on the movement of 14C between the symbionts and they were unable to prevent discs taking up glucose from very dilute (approximately 10−6m) solutions. The rate of movement of glucose between the symbionts was rapid. No penetration of algal cells by fungal haustoria could be detected.