A Necator infection produced by percutaneous exposure of a volunteer to three larvae was followed with periodic egg counts for 4 months beyond 18 years when passage of eggs in the feces ceased permanently. During the 2nd year of infection, there was unintentional exposure to two additional larvae. Egg counts per gram of feces (EPG) were approximately 1,000 during the 1st year, 1,500 over the next 5 years, and ≤200 over the final 3 years, during which time there were 6 periods of approximately 1 month each when Kato thick-smear examinations revealed no eggs. During 6 of the 1st 14 years, when egg counts were done by the standard direct smear method, up to 37% of the eggs were infertile. Based on the assumption that two female worms were present during the 1st year, three during the next 3 years or more, and only one during the final 3 years, the estimated output per female in this light infection was 500 EPG in the worm's prime of life, and less than 200 EPG in the final year of the 17 or 18 year life span.