During a three-week period in 1970 an estimated 65,000 barrels of 34° API gravity crude oil were discharged from the Chevron Main Pass Block 41C Platform, 11 miles east of the Mississippi River Delta. Two thousand barrels of chemical dispersants were sprayed on the platform and surrounding water surface. It is estimated that between 25–30% of the oil evaporated during the first 24 hours, 10–20% was recovered from the water surface, less than 1% dissolved, and less than 1% of the oil was identified in sediments within a 5-mile radius of the platform. The remaining oil emulsified and dispersed to undetectable levels, biodegraded, or photooxidized. The highest measured concentrations in water at the platform and at 1 mile were: oil-in-water emulsion, 70 to 1 ppm; dissolved hydrocarbons, 0.2 to 0.001 ppm; dispersant 1–3 to unmeasurable (<0.2ppm). Total extractable organic matter was highest in sediments near the Mississippi River Delta and in the inland bays. Spilled oil, identified in bottom sediments by gas chromatography, showed rapid weathering after 1 week to 1 month and at the end of 1 year was reduced to a few percent of the amount after the spill. Spilled oil was not found in the sediment below 1.5 inches. Over 550 species of benthic organisms were identified in 233 benthic samples. The number of species and number of individuals of benthic organisms showed low values in some samples near the platform. However, seasonal variations, bottom sediment type, and possibly other environmental parameters made it impossible to determine whether these locations had been affected by the spilled oil. There was no correlation of number of species, number of individuals, or other biological parameters with the hydrocarbon content of the sediments for samples from within a 10-mile radius of the platform. This lack of correlation suggests lack of significant effect of oil on benthic organisms. Extensive trawl samples showed no alteration in the annual life cycle of commercially important shrimp. Blue crabs were observed throughout the area, and the number of species of fish collected were comparable to a prior survey.