Bioassay using Daphnia magna identifies the suitability of amendments which create favorable/toxic conditions to the aquatic organisms. Seed germination and root elongation test is also used to evaluate the hazardous waste sites and to assess the toxicity of organic and inorganic compounds. Therefore, the objectives of this study were to determine arsenic (As) toxicity (1) from soil and As solution towards Daphnia magna and lettuce (Lactuca sativa) in the presence of biochars (poultry manure and hairy vetch biochars prepared at 450 and 700℃) and red soil. Highly contaminated As soil near a mine in Tancheon, Korea was used for the test. The test solution was extracted from soil in the presence of amendments using Milli-Q water. Arsenic solution as arsenite (As2O3-1 and 5 mg L-1) in the presence of biochars was also used in this study. Freshly hatched D. magna (<24 h old) was used for this study. Immobilization of daphnids in each test solution was checked after 48 h. Lettuce seeds were used for the seed germination test in Petri dish and after 72 hours, germination percentage and root elongation were calculated. In soil solution, poultry manure biochar was very effective, especially at high temperature in decreasing the mortality percentage of daphnids. Biochar plus red soil also effectively reduced the mortality percentage of daphnids. While arsenite solution (5 mg/L) caused 90% mortality of daphnids, poultry manure biochar and red soil were very effective in decreasing the mortality percentage of daphnids. The germination percentage did not vary much between the treatments. Significant results were obtained when seeds were placed in As solution (5 mg L-1) in the presence of hairy vetch and manure biochar at 700℃. The root length was very low in control and hairy vetch biochar treatment. However, root length increased greatly when seeds were placed in poultry manure biochar treatment. This study shows the effectiveness of biochar and red soil in the remediation of As-contaminated soils and water. However, depending on the types of biochar and its pyrolysis temperature, results may vary and therefore, careful selection of amendments that are safe for plants, soil biota and aquatic organisms should be made.