The degree to which infectiveness to intermediate hosts is retained as eggs of Moniliformis moniliformis (Acanthocephala) age was studied by determining the prevalence and mean intensity of cystacanths produced in the cockroach (Periplaneta americana) intermediate host. Eggs up to 9 wk old resulted in more prevalent and intense infections than did older eggs. After this initial flush of infectiveness, prevalence and mean intensities did not decline gradually to uninfectiveness, but rather they quickly fell to a relatively stable level that was maintained for another 69 wk before eggs became uninfective. Thus, except initially and at the end of infectivity, there was a period of more than a year when the age of eggs played no role in determining their infectivity. No infection was produced by eggs older than 82 wk.