An extracellular esterase, EstK, was purified from the psychrotrophic bacterium Pseudomonas mandelii grown at 25A degrees C. Prior to harvest, cells were treated with 0.2 M MgCl2 to precipitate lipopolysaccharides in the outer membranes, which otherwise form aggregates with the secreted enzymes. EstK was purified to homogeneity using standard procedures. It had substrate specificity towards esters of short-chain fatty acids, particularly, p-nitrophenyl acetate. Optimum activity of EstK was at 40A degrees C; at 4A degrees C the activity was similar to 50% of its maximum. EstK has a unique substrate preference for p-nitrophenyl acetate and remains active at low temperatures.