δ13 values were determined for leaves of C4 grasses (Poaceae) of different photosynthetic types. For plants grown in the same environment at the same time, mean δ13 values for the three C4 types were: NADP-ME-type, - 11.350 ± 0.13 s.e. (11 spp.); PCK-type, - 11.950 ± 0.19 s.e. (11 spp.); and NAD-ME-type, -12.70 ± 0.21 s.e. (9 spp.). Although there is some overlap between the values for individual species of the three groups, the difference between any two means is highly significant [P(t) < 0.01] and is not due to taxonomic sampling bias at the subfamily level. The differences in means may suggest that C4 types differ in rates of leakage of CO2 and HCO3- from PCR tissue ('photosynthetic carbon reduction' tissue: equivalent to 'Kranz' tissue), and/or, using Farquhar's (Appendix) expression for plant δ values, that C4 types differ in their average intercellular CO2 concentrations (c1). It is also possible that differences between C4 types exist in some other, unknown, leaf fractionation process. Apoplastic CO2 leakage from PCR tissue in NAD-ME-type C4 grasses, which do not possess a PCR 'suberized lamella' as found in NADP-ME- type and PCK-type C4 grasses, may give these species the most negative δ13 values. Expressions for C4 plant δ13 values, and a model for the δ13 values of CO2 and HCO3- in various pools and fluxes in C4 plant leaves, are given.