The rapidity with which hookworms are lost from the intestine of a human host in the absence of reinfestation is a question of great practical importance. If, as Smillie1 predicated in 1922, hookworms are slowly acquired and slowly lost, it would mean that improvement in the level of hookworm infestation would follow sanitary reforms rather slowly, whereas such improvement would follow rapidly and have a long-lasting effect after anthelmintic treatment. It would mean also that even in countries with a dry season of from four to six months' duration a mass treatment at any time of the year would be effective, whereas the effect of the dry period during which reinfestation was practically stopped would be of no practical consequence beyond reducing the number of months of the year during which hookworms could be acquired to six or eight instead of twelve. If, on the other hand, hookworms are