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Abstract. Estimates were made of the number of Giardia in 1,090 stools from 15 infected children over periods of 1 to 3 months. Three patterns of excretion were observed: 1) high, with the parasite abundant in nearly all stools; 2) low, with the parasite detectable in only 40 % of stools and scanty when present; and 3) mixed, with periods of 1 to 3 weeks of high excretion alternating with generally shorter periods of low excretion, and an overall average of about 60 % of stools positive. The presence and relative numbers of Giardia in the feces apparently were unrelated to either the consistency of the stools or frequency of defecation. Attempts to increase parasite excretion with purgatives were, on the whole, unsuccessful. Giardia lamblia infection is often difficult to detect and stool examinations frequently yield negative results that are questionable. Observing experimental Giardia infection, Rendtorff found some people to have periods of no parasite ex cretion interspersed with periods of high parasite excretion, whereas in others nearly all stools examined were positive. ' In a survey of 10