Biochars were prepared by pyrolyzing pig manure, cow dung, bamboo, corn straw, and sewage sludge at 400 and 600 degrees C; then, the environmental risks posed by heavy metals and polycyclic aromatic hydrocarbons in the biochars were assessed. The acid-extractable and reducible fraction contributions to the total Zn concentrations were 53.78% and 46.11% for cow dung biochar, 51.94% and 48.37% for sewage sludge biochar, and 13.51% and 7.99% for corn straw biochar prepared at 400 and 600 degrees C, respectively, that is, lower after pyrolysis at 600 than 400 degrees C. The polycyclic aromatic hydrocarbon concentrations were 1376.09-2695.74 mg kg-1 for biochar prepared at 400 degrees C but much lower for biochar prepared at 600 degrees C. Superoxide dismutase and carboxylesterase activities were higher for Chironomus sp. larvae exposed to biochar than blank larvae, indicating that the biochars were toxic. The superoxide dismutase activities were 33.8 times higher for cow dung biochar prepared at 400 degrees C than the blanks. Multivariate statistical analysis indicated that extractable Cd and Cu in biochar cause aquatic toxicity. The bioavailabilities and toxicities of heavy metals in animal manure and sewage sludge decrease through pyrolysis to give biochar but heavy metals may remain toxic to aquatic organisms.