The high concentrations of short-chain soluble polyphosphates, previously reported in Phytophthora palmiwora, have been found in six other species of Phytophthora, and appear to be characteristic of this genus. The distribution of phosphorus among the various acid-soluble pools was similar in all species except P. infestans, which accumulated less polyphosphate than other species when grown in high phosphate medium, and contained several unique and as yet unidentified organic phosphorus compounds. When grown in the presence of potassium phosphonate sufficient to reduce growth b y 50%, all species showed an increased accumulation of phosphorus in both pyrophosphate and polyphosphate, without parallel increases in sugar phosphate or nucleotide phosphorus pools. The internal concentration of phosphonate required to produce 50% inhibition varied with species, ranging from 4-6 to 52 pmol (g dry wt-'. Assimilation of orthophosphate was only slightly reduced in the presence of phosphonate, resulting in an increased concentration of phosphorus per unit mass. Metalaxyl, which also inhibits growth of Phytophthora spp. did not cause accumulation of either pyro-or polyphosphate. Phosphonate treatment also resulted in the formation of a compound identified as isohypophosphate, a metabolite of phosphonate not previously reported in Phytophthora. Taken together, these observations suggest that the primary site of phosphonate inhibition in Phytophthora spp. lies in the metabolism of pyrophosphate.