ABSTRACT
Several nutrient agar plates with red colonies were dispathched to the laboratory from Tehran milk pasteurization plant which were cultured from milk samples. To determine the microbe, the colonies were streaked on the nutrient agar and incubated at 30°C for 24 hrs. The red round and smooth colonies with 1-2 mm in diameter developed on the agar surface which were transferred on TSI tubes and incubated at 30°C for 24 hrs. The result was yellowish [acid] reaction through the TSI tube with red pigment producing on the top of the tube. The smears were prepared and gram stained and polymorph gram negative bacteria observed which were analyzed by biochemical test such as lactose, Rhamnose, Sucrose, Citrate, Nitrate, Pepton water, MR-VP, Gelatine and Motility. The bacterium was identified Serratia marcescens which has an important role in the spoilage of the food e.g. milk and milk products and produces red pigments on the butter an other long life dairy products