Abstract: In this paper, a new drilling mud additive produced as a microbial exopolysaccharide by Brevibacterium viscogenes 74-230 from a heavy liquid paraffin is reported. Addition of the surfactants of Tweens or Span-80 to the medium can promote polysaccharide biosynthesis, but penicillin G and potassium ion has no advantageous effects on it. Corn steep liquor can be used as a substitute of yeast extraet, but with poorer results. After removal of cells by centrifugation, over 12g/l of polysaecharide can be obtained by ethanol precipitation, at a conversion rate based on liquid paraffin in exce'ss of 40%. Its rheological property, shear thinning effect, water loss, sensitivity to salts as well to as temperature, etc. are determined and compared with xanthan, partially hydrolysis polyacrylamide and carboxylmethylcellulose. The results indicate that the properties of this polysaccharide as a drilling mud additive are almost comparable with those of Xanthan, and far better than those of the other two. It is noteworthy that this polysaccharide is produced from crude oil or its product, while Xanthan is from carbohydrate. Field experiments are still going on.