A screening work was carried out to get marine bacterium with high alginate lyase activity by using alginate as sole source of carbon.Strain QM42 was selected for further studies among the 8 alginate-degrading bacteria strains.Results showed that the optimal carbon and nitrogen sources for marine bacterium QM42 to produce extracellular alginate lyase were alginate and peptone,respectively.The optimal concentration of carbon source was 0.4%~0.7%(w/v).The culture temperature was 31℃.The initial pH value of the liquid medium was about 7,and the salinity was 5%~7%.The optimal harvest time for extracellular alginate lyase was about 72h with 150r/min rotation.Under such conditions the marine bacterium QM42 could produce 9.74U/ml alginate lyase in crude fermentation medium.