A description of ecological effects harmonization activities occurring since 1990, within the U.S. EPA's Office of Prevention, Pesticides, and Toxic Substances (OPPTS), and occurring between OPPTS and OECD was presented at the ASTM Eighth Symposium on Environmental Toxicology and Risk Assessment in 1998. This presentation was subsequently published (Smrchek and Morcock 1999). Recently, there has been a great increase in harmonization activities between OPPTS and OECD. Many additional activities are currently ongoing in five test method areas: physical chemical properties (e.g., Kow methods), effects on biotic systems (ecological effects, e.g., Lemna growth inhibition, covered elsewhere), fate (degradation and accumulation, e.g., aerobic and anaerobic biodegradation), health effects, and in special activities such as in method development and testing of endocrine disruptors (e.g., avian reproduction, fish whole life cycle). The goal remains as before: to harmonize OPPTS guidelines (eventually to be published final as parts 810–885 in volume 40 of the U.S. Code of Federal Regulations) and test guidance with OECD so as to avoid unnecessary and duplicative testing of those chemicals marketed on a world wide basis, to increase efficiency, save test animals, and to reduce nontariff barriers to trade. The latest harmonization activities and results achieved, with a specific emphasis on aquatic and sediment methods are presented. ASTM has played and must continue to play an important part in the U.S. role in the development of these OECD tests. Recent activities include revision of OECD Test Guideline No. 202, Daphnia sp., Acute Immobilisation Test; development of a new oligochaete reproduction test; two chironomid sediment tests (spiked water and spiked sediment); an oligochaete bioaccumulation test; and a guidance document on Aquatic Toxicity Testing of Difficult Substances and Mixtures. Challenges and potential problems for effective harmonization continue to be present. These can be categorized into internal within the U.S. EPA and external to the agency. The former include having adequate available resources, redistribution and changes in workloads to accommodate OECD work, continual education to demonstrate the advantages of harmonization, and obtaining continued support in an environment of budget constraints. External aspects include challenges from the public, the problem of incurring continuing harmonization costs by U.S. industry, accommodating voluntary standards organizations such as ASTM, and difficulties in developing a “U.S. position” on OECD test guidelines. There also continue to be difficulties (not unexpected) in coordination, communication, and cooperation in expert panels and among participating OECD member countries. Several principles for effective harmonization in updating existing OECD test guidelines or in developing new test guidelines are discussed.