Stomatal responses to light of Arabidopsis thaliana wild-type plants and mutant plants deficient in starch (phosphoglucomutase deficient) were compared in gas exchange experiments. Stomatal density, size and ultrastructure were identical for the two phenotypes, but no starch was observed in guard cells of the mutant plants whatever the time of day. The overall extent of changes in stomatal conductance during 14 h light–10 h dark cycles was similar for the two phenotypes. However, the slow endogenous stomatal opening occurring in darkness in the wild type was not observed in the mutant plants. Stomata in the mutant plants responded much more slowly to blue light (70 μmol m−2 s−1) though the response to red light (250 μmol m−2 s−1) was similar to that of wild-type plants. In paradermal sections, stomatal responses to red light (300 μmol m−2 s−1) were weak for wild-type plants as well as for mutant plants. Stomatal opening was greater under low blue light (75 μmol m−2 s−1) than under red light for the two genotypes. However, in mutant plants, a high chloride concentration (50 mol m−3) was necessary to achieve the same stomatal aperture as observed for the wild-type plants. These results suggest that starch metabolism, via the synthesis of a counter-ion to potassium (probably malate), is required for full stomatal response to blue light but is not involved in the stomatal response to red light.