Nine hundred fifty male Hubbard chicks were used in a 21-d study (10 birds per battery cage) to determine:relative bioavailability of P (RBP) in four feed-grade phosphates (FP) [two Brazilian dicalcium and two U.S.-made phosphates (di-monocalcium and defluorinated)] and four Brazilian agricultural grade phosphates (AP) [single (AP-1), and triple (AP-2) superphosphates, monoammonium (AP-3), and thermomagnesium (AP-4) phosphates]. The reference standard was a purified-grade calcium phosphate dibasic (SP). Phosphates were added to the corn-soybean control diet (22% protein; 0.40% P + 0.08% P from SP), providing 0.08 and 0.16% additional P. Calcium level was 1.0% for all diets. Slope ratio was used to determine REP, with BW, bone ash (BA), or bone strength (BS) regressed on P added within each P source. A relative biological value (RBV) was estimated using BW, BAI and feed efficiency. Performance was depressed (P <0.01) by AP as compared with FP; BW was decreased by 11%, and feed intake (FI) was decreased by 14%. Mortality increased (P < 0.05) by 154% (7 vs 2.8%). Phosphate source:AP-4, which had the lowest content of P and a high content of F, Fe, Ba, Ti, and Th, was toxic based on a 44% decrease (P < 0.01) in BW, 46% decrease in Fl, 19% decrease in BA (32.4 vs 40.0%), 55% decrease in BS (7.1 vs 15.8 kg), and mortality increase (P < 0.05) from 0.7 to 26% compared with the average of AP-1, -2 and -3. The REP could not be estimated for AP-4; and average availabilities for FP and AP, respectively, were 100.6 and 107.6% (BW), 88.3 and 93.2% (BA), 84.2 and 96.3% (BS), and 100.0 and 99.9% (RBP). The AP varied in REP, with particularly high values calculated for AP-3. Performance and bone parameters in this study were not strongly affected by high levels of potentially toxic mineral elements in certain AP; this result may be explained by the low levels of phosphate addition and the short duration of the-feeding period (21 d). However, considering their relatively high levels of F, Fe, Mg, S, Ba, Ti, and Th, agricultural-grade phosphate may represent considerable risk of toxicity for use in animal diets.781217291736