Waterlogged soils are typically anoxic, whereas roots require oxygen. Roots of most wetland plants contain aerenchyma and many species also possess a barrier to radial oxygen loss (ROL). Restriction of ROL from the basal zones promotes oxygen movement along roots to the tips. In addition to being anaerobic, ‘phytotoxins’ such as Fe2+, sulphides and monocarboxylic acids can accumulate to toxic levels in waterlogged soils. These phytotoxins can invoke marked changes in root anatomy, radial permeability and growth. We tested the hypothesis that low, non-toxic, concentrations of monocarboxylic acids can induce root ROL barrier formation in rice (Oryza sativa). Acetic, propionic, butyric and hexanoic acids, individually at 0.038-0.048 mM (undissociated component), each triggered root ROL barrier development. Root extension was unaffected by acetic or butyric acids, but mildly inhibited (at most 10%) by propionic or hexanoic acids, at these concentrations. Root respiration of expanded tissues was not, or in some cases moderately, inhibited by short-term exposures to these concentrations of monocarboxylic acids, whereas respiration declined exponentially as the dose was increased. Thus, low concentrations (0.038-0.048 mM) of four monocarboxylic acids can induce root ROL barrier formation in rice, a response apparently independent from phytotoxic effects on root extension.