The ni trogen fix ation in shrimp culture enclosures at dif ferent culture sty les w as measured using acethlene reduction method .The results were as follow s :In all enclosures , the nit rogen fixation rates w ere the highest 2 .62 μgN/(L·hr)fo r Y -4 , the enclosure monocultured w ith shrimp Penaeus chinensis and 2 .65μgN/(L·hr)for Y -5 , the enclosure polycultured w ith shrimp and bay scallop , the lowest 1 .53μgN/(L·hr)for T -5 , the enclosure polycultured w ith shrimp, tilapia Oreochrromis niloticus and const racted tanglus Sinonovacula costricata , 2 . 30μgN/(L·hr)for Y -6 , the enclosure polycultured wi th shrimp and constrcacted tang lus, and 1 .89μgN/(L ·hr) for Y -7 , the enclosure polycultured with shrimp and tilapia .In the enclosures, the high ni trogen f ixatio n rates may co ncern with the low avaliable nitrogen concent ration and the low N/P ratio , compared w ith the Redfield rat io .The nit rogen fixation blue-g reen algae were mainly Chroococus , and then Nostoc .Estimated nit rogen fixation as nit rogen input s for dif ferent enclosures was :17 .71g for Y -4 , 19 .63g for Y -5 , 90 .5g for Y - 6 , 10 .59g for Y -7 and 7 .99g for T -5 , respectively .