The chemical methods for identifying and assessing Acid Sulfate Soils (ASS) have now been reasonably developed. However, there is a missing link between the chemical criteria set in ASS guidelines and the ecotoxic effects of contaminants that are likely to be exported to the aquatic environments from ASS. Acid sulfate water consists of a mixture of potential toxicants. It is therefore unreliable to assess the overall toxicity of an acid sulfate effluent solely based on the measurements of chemical parameters because the combined toxicity of various toxicants may be substantially greater than the sum of individual toxicities. To better control the ASS-related environmental degradation, it becomes necessary to implement the chemical analysis and monitoring with some sort of ecotoxicological assessment. We have developed methods for ecotoxicity testing of ASS using D. carinata, a microinvertebrate that is naturally associated with ASS-related aquatic environments. Like the universally adopted Daphnia magna, D. carinata is able to reproduce by cyclical parthenogenesis, be easily cultured in the laboratory, and be highly sensitive to toxicants. However, D. carinata is superior to D. magna as a test organism for ecotoxicological assessment of toxicants. The major advantages of D. carinata (Clone DC42) over D. magna include: (a) a wider range of temperature tolerance (–4oC - 38 oC, compared to 5oC - 28 oC for D. magna); (2) rapid growth rate and shorter cycle of reproduction (5 days, compared to 15 days for D. magna); (c) more sensitive to toxicants; (d) stronger phototaxis; and (e) a wider food range. These advantages make the D. carinata (Clone DC42) an ideal biotoxicity test organism. An integrated biomarker approach was used for ecotoxicolgical evaluation of ASS. Bioassays suitable for routine ecotoxicity tests of ASS extracts or leachates were developed using mobilization inhibition, mortality and phototactic behaviour of D. carinata as test endpoints. A criterion known as median effective dilution factor (EDF50) was proposed for ecotoxic level classification of ASS, which allows toxicity screening of ASS at the early stage of ecotoxicological assessment of ASS-affected environments