SUMMARYThe population dynamics of Ascaridia galli was studied in 70 ISA Brown layer pullets, 42 of them were each experimentally infected with 500 embryonated A. galli eggs and 28 chickens were kept as uninfected controls. Six chickens from the infected group and 4 from the control group were necropsied at 3, 7, 10, 14, 21, 28 and 42 days post-infection (d.p.i.). The mean worm recovery varied from 11–20% of the infection dose with the highest recovery at 3 d.p.i. and the lowest at 21 and 42 d.p.i. (P < 0·05). More larvae were recovered from the intestinal wall than from the content (P < 0·0001) and intestinal content larvae were longer than those from the wall (mean length 1·6 and 1 mm, respectively, P < 0·0001). Although larvae were growing over time, a population of small-sized larvae (length < 1 mm) was recovered at all d.p.i. During the first week of infection most of the larvae were located in the anterior half of the jejunoileum but they moved posteriorly with the age of infection. Thus, a subpopulation of larvae mainly in the lumen grew with time while another subpopulation remained small and associated with the mucosa. During the infection both subpopulations moved to a more posterior localization in the gastrointestinal (GI) tract.