An epidemiological survey of intestinal parasitic infections was conducted in a sample of 203 children aged 3–5 years from a semi-urban and a rural community in Chiriqui, Panama, in 1983–4. On the basis of stool examinations, the prevalences of Ascaris lumbricoides, Trichuris trichiura, hookworm, Giardia intestinalis, Entamoeba spp. and Strongyloides stercoralis were found to be 27, 34, 14, 15, 5 and 4% respectively. The results from children from the two communities were compared. Polyparasitism occurred significantly more often in rural than semi-urban children. Following anthelmintic treatment with levamisole, the numbers of A. lumbricoides passed/child were recorded and the frequency distribution of the parasite was observed to be highly aggregated with a variance to mean ratio of 10·2. For A. lumbricoides, relationships between worm burden, worm biomass and egg production were investigated. In the data analysis, an attempt was made to explore the influence of numbers of male worms on egg production. The results are compared with those obtained during other recent studies on the epidemiology of A. lumbricoides infection in other countries.