Acute benthic toxicity tests can be a useful tool for screening nonaqueous base fluids used in drilling fluids. These toxicity tests were originally designed to evaluate dredged spoil, and were adapted and introduced as a standard benthic test, using field sediments for approval of nonaqueous ( ) drilling fluids, by the Oslo & Paris Commission OSPAR in the North Sea in 1994. In the US, development of a standard benthic test for regulatory approval of nonaqueous drilling fluids and screening new base fluids is continuing. There is now some concern that the variability and nature of field sediments can cause effects in addition to the toxicity of the test material. Use of formulated sediment to replace natural sediment may greatly decrease this variability, and allow for greater interlaboratory comparisons. The formulated sediment developed and refined in this study consisted of 30% sand, 20% silt, 48% clay, 1% organic material, and 1% calcium carbonate. This formulation ( ) produced acceptable control survival )90% in ten toxicity tests using Leptocheirus plumulosus. The results of five diesel toxicity tests and five C1618 internal olefins toxicity tests produced coefficient of variations of 16.0 and 28.9, respectively, indicating the formulation can produce consistent test results.