To compare the structures of natural and re-established populations of Ascaris in humans, universal (mass) chemotherapy was carried out at the beginning and the end of the study year using pyrantel pamoate. Worms expelled within 48 h of treatment were collected, their sex determined, and measurements made of length, width and weight. Length was used as the criterion for estimating the developmental stage of the worms. In comparison with the natural population, the re-established population displayed similar sex ratio as well as distribution patterns among individuals and age groups of the host. However, the mean worm burden of the re-established population was significantly decreased, with a reduction of burdens in children aged 5–9 years. Also, the re-established population showed significant changes in population structure and worm measurements in that it comprised more immature and less fertile males, less fertile and more senile females, smaller and lighter males, larger (but not heavier) females than the natural population. The results suggested that the re-established Ascaris population did not restore to its original status in relation to mean density, composition and fecundity. Therefore, universal treatment once a year should decrease the transmission of Ascaris in humans. Combined with previous results for the same study sites, the present findings also indicated that caution is warranted to avoid misleading conclusions when using prevalence and faecal egg counts as parameters for evaluating the success of control programmes.