We present a case of Ascaridia galli infection in laying hens on a farm in central Serbia. During the rearing period on litter, ascaridosis was diagnosed at 15 weeks of age by routine parasitological fecal examination. Pullets were treated with flubendazole for one week, and two weeks later the hens were moved to battery cages. The production results were within technological standards until the 23rd week and the medical health status was without any clinical symptoms. After that period weight loss began, the egg production dropped to 70% and eggs were of poor quality. Subsequently, severe feather pecking and an increase of mortality were reported. The postmortal examination showed severe anemia and several gross lesions in the liver, intestines, lungs, and kidneys. Different sizes of A. galli were found in the lumen of the duodenum and jejunum. Gross changes of the intestinal mucosa were present, such as a thickened intestinal wall with hemorrhagic spots, inflammation and necrotic patches. Histopathological examination showed marked changes in the intestines, liver and kidneys. All visible live parasites were collected and stored in Earle's balanced salts, and females were used for in vitro susceptibility testing. Median lethal concentration (LC50) of piperazine, levamisole and carvacrol for A. galli was 119.7 mu M, 2.71 mu M and 3.26 mu M, were applied, respectively. Based on our results, it is likely that reinfection occurred after completed dehelmintization. In relation to the new circumstances and the regulation for laying hen welfare the deworming protocol should be changed in order to ensure successful dehelmintization. In order to prevent reinfection the treatment must be done at the end of the rearing period and thus be maximally effective.