The extracellular alginate lyases were purified from Vibrio harveyi AL-128 and V. alginolyticus ATCC 17749. The former enzyme appears to be specific for alpha-1,4 bonds involving L-guluronate units in alginate, whereas the latter exhibits specificity for beta-1,4 bonds involving D-mannuronate units. The molecular weights of the enzymes were estimated to be 57,000 and 47,000, and they had isoelectric points of 4.3 and 4.6, respectively. The enzyme from strain AL-128 was most active at NaCl concentrations of 0.3 to 1.0 M. Optimum activity of the enzyme from strain ATCC 17749 was found in the presence of 5 to 10 mM CaCl2.