Abstract: Morphological changes of Ancylostoma duodenale and Necator americanus at various development stages in animal hosts (young dogs and golden hamsters) were studied.Both species during their skin and muscle stages are similar to the 3rd stage larvae before entering hosts.In host s lung,N.americanus undergoes some obvious changes but not A.duodenale.In the 4th stage larvae,the length of mouth cavity of A.duodenale is almost equivalent to its width,but that of N.americanus is shorter.At that stage,the vulva of N.americanus is situated at the posterior part of its body;while in the 5th stage larvae and adult,it is at the anterior part.There is a pair of openings on the sub-ventral part of mouth cavity in both species,but more prominent in A.duodenale.Another pair of openings occur in the deep sub-dorsal part of the mouth cavity,more prominent in N.americanus.At the 4th stage,besides tue vulva,worm sex can also be distinguished by its tail.A pair of spiral phasmids are found in the anterior ventral part of bursa.Two pairs of papillae on the dorsal surface of either side of closcal openings are found in both species,but obvious differences in morphology between two species.In addition,it is observed for the first time that there is a pair of small papillae in the position of lateral ray of bursa in male and a tail spicule in female of N.americanus.