Discs (0·6 cm diameter) from submerged leaves of Potamogeton lucens L., P. perfoliatus L., P. polygonifolius Pourr. and P. praelongus Wulff were allowed to photosynthesize in well stirred conditions at pH 4·3, 20 °C, and a saturating light intensity of 120 W m−2 (400 to 700 nm) in a range of free CO2 concentrations for 1 h, and the rates were calculated from 14CO2 uptake. Total resistance (1/k+ 1/P) was calculated from these data, using a modification of Fick's first law. 1/k+ 1/P was found to vary between 3·7 × 104 and 5·9 × 104 s cm−1 in the four species. The internal diffusive resistance (l/D) only made up 2·6 to 4·3 % of this total. Estimates were made of the proportions of 1/k (the chemical resistance), 1/P (the diffusive resistance) and δ/D (the boundary layer diffusive resistance) to the total resistance over a range of possible values for the concentration of CO2 at the site of carboxylation: 1/P was always greater than 1/k, and δ/D was always the single largest resistance. The average boundary layer thickness for the leaf discs was calculated as 340 μm; for whole leaves of these species, also well stirred, the boundary layer thickness was estimated to range between 440 and 1090 μm. The ecological significance of the boundary layer to freshwater macrophytes in standing waters is discussed.