Adults of Ascaris lumbricoides were recovered from 252 persons in the village of Jazin near Esfahan, Iran after treatment with pyrantel pamoate. This horizontal study provides information on age-specific prevalence rates, intensities of infection, frequency distributions, sex ratios and eggs per gram of stool at different parasite intensities. Recruitment of A. lumbricoides was then monitored at 7, 30, 60, 90 and 365 days after treatment in 110 persons. A separate sample population was examined at monthly intervals and, in addition, the stool of each member of a nine-person family was examined for 10 consecutive days. These data are analysed and used to describe the dynamics of transmissions and recruitment of A. lumbricoides in Jazin. The population dynamics of ascariasis are discussed in the context of R the basic reproductive rate of the parasite. The relationships of R with frequency distributions, prevalencies and worm burdens are discussed. Because of the considerable potential of A. lumbricoides to re-infect man in endemic areas, it is shown that it will be difficult to eradicate ascariasis by the use of chemotherapy, even when drugs with a high efficacy are used repeatedly.