Bean (Phaseolus vulgaris L.) seedlings were cultured on complete or phosphate-deficient nutrient medium. After 14 days of culture on phosphate-deficient medium the visible symptoms of Pi deficiency were observed only in the shoot, the fresh and dry weights of the roots were slightly higher than in control plants. The decreased Pi content in the roots had little effect on total respiration rate but had an effect on the level of inhibition of respiration by cyanide. The high resistance of respiration to cyanide observed in Pi-deficient roots was the result of the suppression of cytochrome path activity and an increased participation of the alternative, cyanide-resistant pathway. The cytochrome pathway activity increased when inorganic phosphate was supplied to Pi-deficient roots for 1 or 3.5 h. It is speculated that the suppression of cytochrome pathway in Pi-deficient roots may result from restriction of the phosphorylating capacity or a partial inhibition of cytochrome oxidase activity.