Phosphite (; Pi) uptake in cell suspension culture, information on how Phi affects the Pi uptake of intact plants remains to be determined. The present study was conducted to investigate the effect of Phi on Pi absorption of intact komatsuna plants (Brassica rapa var. peruviridis cv. Ajisai) in hydroponic culture. Phosphite markedly decreased Pi absorption of the intact komatsuna plants under both low (0.05 mmol L−1) and high (0.5 mmol L−1) Pi supply, although the growth (both shoots and roots) and water uptake of the high Pi-supplied plants was not affected by Phi. The inhibiting effect of Phi was small at 0.2 mmol L−1, but became large at 2 mmol L−1. Using relatively large seedlings (28 days old) to better assess the influence of Phi on Pi absorption early in the treatment, the results indicated that there was an immediate decrease in Pi absorption within the first 2-day period of Phi treatment when the water absorption of the plants was not affected. Taken together, the results suggested that there was a strong inhibiting effect of Phi on Pi uptake of intact komatsuna plants and this effect is exerted most likely by competition between Phi and Pi at uptake level. We speculate that the application of Phi to plant roots in an environment that is unfavorable for Phi-to-Pi conversion (e.g. hydroponic culture) may need to increase the amount of required Pi fertilization of plants to compensate for the reduction in Pi uptake by Phi. Further research is needed to confirm our results.