AbstractMeasurements are presented ofδ13C values from nineSphagnumspecies and associated vascular plants and peat from a wet heath in East Devon, fromSphagnum trinitenseC. Müll. at different sites and heights above water level in a North Carolina lake, and from two Marchantialean and two Metzgerialean liverworts from various habitats. The terrestrialSphagnumspecies show broadly similarδ13C values. Species with photosynthetic cells enclosed by the hyaline cells on both surfaces have significantlylowerδ13C values (mean -27.12) than the other terrestrial species (mean - 28.91).S. cuspidatumfrom a bog pool has aδ13C of -34.70, very significantly more negative than the other sphagna, probably from a proportion of recycled CO2in its source carbon.S. trinitenseshows significant between-site and within-site variation inδ13C in relation to height above water level. Samples at 6 cm give the most negativeδ13C values (mean -26.10), with progressively less negative values at lower levels (mean at 3 cm, -23.98; submerged, -22.55), implying increasing limitation of carbon uptake by diffusion resistance with increasing saturation and submersion.The four liverworts give similarδ13C values. The slightlymore negative figure forAneura pinguismay reflect some uptake of recycled CO2; the figure forPellia epiphyllais close to that for the two Marchantiales.Bryophytes have generally similarδ13C values to C3 vascular plants, suggesting that their photosynthetic structures are functionally comparable to those of vascular plants, with similar relations between diffusive and biochemical limitations on the rate of carbon uptake. This probably results from similar selection pressures operating in the two groups. Relationships between structure, diffusion resistances and inorganic carbon acquisition inSphagnumare discussed.