Acinetobacter radioresistens KA53, isolated by enrichment culture, was found to produce an extracellular, nondialyzable emulsifying agent (referred to as alasan) when grown on ethanol medium in a batch-fed reactor. The crude emulsifier was concentrated from the cell-free culture fluid by ammonium sulfate precipitation to yield 2.2 g of emulsifier per liter. Alasan stabilized a variety of oil-in-water emulsions, including n-alkanes with chain lengths of 10 or higher, alibi aromatics, liquid paraffin, soybean and coconut oils, and crude oil. Alasan was 2.5 to 3.0 times more active after being heated at 100°C under neutral or alkaline conditions. Emulsifying activity was observed over the entire pH range studied (pH 3.3 to 9.2), with a clear maximum at pH 5.0. Magnesium ions stimulated the activity both below (pH 3.3 to 4.5) and above (pH 5.5 to 9.3) the pH optimum. Alasan activity was higher in 20 mM citrate than in 20 mM acetate or Tris-HCl buffer. Preliminary chemical characterization of alasan indicated that it is a complex of an anionic, high-molecular-weight, alanine-containing heteropolysaccharide and protein.