Root hair development is influenced by nutrient availability. In particular, it has been described that nitrogen and phosphorus concentrations influence secondary root formation and root hair development. In Arabidopsis it has been reported that low P concentrations stimulate root hair formation and/or elongation. To determine at which level P interferes with root hair development, we have analyzed the effect of different concentrations of P in different Arabidopsis genetic backgrounds. Using root hair defective Arabidopsis mutants, we determined that P interferes with root hair elongation but not with root hair formation. We also found that P is able to suppress root hair elongation in mutants that form ectopyc root hairs and that auxins and the ethylene precursor ACC are not able to suppress the root hair elongation inhibitory effect of high P concentrations.