Summary δ13C measurements were made of dissolved inorganic C, and of submerged benthic cyanobacteria, algae and bryophytes, from Allt Meall nan Damh, a burn at Ardeonaig, Perthshire. The δ13C of the CO2, HCO3/- and CO3/2- components of the inorganic C were computed, and the Δ values of the organic C in the photolithotrophs were then calculated relative to dissolved CO2. The decreasing order of A values in the Ardeonaig Burn is Lemanea and bryophytes ≥ green macroalgae and Audouinella > diatom mats, which is the same as in the Dighty Burn. However, the Δ values of Lemanea and the bryophytes, which depend on diffusive CO2 entry, are lower at Ardeonaig than in the Dighty Burn, suggesting greater diffusive limitation to photosynthesis in the Ardeonaig Burn. It is not easy to relate this difference in Δ values in Lemanea to the higher C:N atomic ratio in the Ardeonaig Burn (21.2 ± 0.64) than in the Dighty Burn (9.5–11.0). The Δ values relative to HCO3/- for the HCO3/--using diatom mat in the Ardeonaig Burn is ...