The toxicity evaluation of drilling fluids using Gulf Coast mysid shrimps has become well developed during the past several years and is used by the Region II EPA to evaluate discharges from offshore platforms in the mid-Atlantic. Development of a method to utilize native mysid shrimps for Pacific Ocean platforms, however, is still in its infancy. A new preliminary acute toxicity method has been developed using Acanthomysis sculpta, a mysid shrimp commonly found associated with central California kelp forests. The new method is similar to mid-Atlantic procedures but has been modified to accommodate the life-style of the Pacific Coast species and to be in compliance with the specifications of EPA, Region IX. A slurry is prepared by combining one part of drilling fluid with nine parts of filtered natural seawater. This solution is mixed on a stir plate while being vigorously aerated for 30 min. The slurry is then allowed to settle for 1 h, and the supernatant is decanted and used directly in the toxicity test. Six 1-L concentrations of the extract solution (usually 100, 56, 32, 18, 10, and 0%) to seawater are prepared in triplicate and placed in a cooling bath maintained at 15.0 ± 1.0°C. Concentrations are aerated as needed to maintain 65% of oxygen saturation, and then 20 previously collected and similar-sized (6 to 8 mm) mysid shrimp are randomly placed in each container. The test continues for 96 h, after which the live organisms are counted and the LC50 interpolated from averaged percent mortality.