Abstract: Two probiotic bacteria,A40209CDC4(Bacillus subtilis)and A31009NA(Sphingomonas paucimobilis),separated from the microflora in the intestinal duct of cultured European eel(Anguilla anguilla)were used to study the microbial colonization and succession.After being fed once per day the diet containing the two microorganisms for 5 days,the tested eels were asceptically dissected for bacteria isolation from their digestive canal at the 1st,2nd,3rd,4th,5th,7th,9th,15th,20th and 30th days.The result showed that(a) A40209CDC4 could colonize in the stomach for at least 7 days and in the intestines for at least 17 days;(b) A31009NA could colonize in the stomach for at least 4 days and in the intestines for at least 9 days;and(c) the changes of the total bacterial counts in the digestive tracts and those of the A40209CDC4 or A31009NA were parallel.The result indicated that A40209CDC4 and A31009NA could colonize well in the digestive canal of the eel,and could transfer from the stomach to the intestine;the microflora in the intestinal duct of the cultured fish could be considerably affected by feeding the fish with the probiotic bacteria.