Technical Abstract: The objective of this study was to determine whether 2 distinct chlamydial isolates recovered from the intestines and feces of diarrheic nursery pigs could cause intestinal lesions in gnotobiotic pigs. Chlamydial isolates R27 and R19 were propagated in Vero cells or embryonated eggs. Each piglet was fed 1 ml of inoculum or sham inoculum at 3-4 days of age. Ten piglets were each fed 10**9 inclusion-forming-units (IFU) and 14 piglets were each fed 10**6 IFU of isolate R27. Twenty piglets were each fed 10**5 IFU R19. All infected piglets developed diarrhea 4-5 days postinfection (DPI). Most piglets fed 10**9 IFU R27 became anorexic, dehydrated, and weak and were necropsied 4-7 DPI. Diarrhea persisted in the piglets fed 10**6 IFU R27 or 10**5 IFU R19 through 12 DPI. At necropsy, all diarrheic piglets had watery colonic contents with flecks of undigested curd. Histologic lesions were seen most consistently in distal jejunum and ileum. Distal jejunum and ileum from piglet fed 10**9 IFU R27 and necropsied 4-5 DPI were characterized by villus atrophy and multifocal necrosis of villi. Mild to severe villus atrophy, lymphangitis, and perilymphangitis were seen in the distal jejunum and ileum from all infected piglets 7 and 10 DPI. Immunostaining done on sections of distal jejunum and ileum revealed chlamydial antigen in villus enterocytes. Ultrastructural examination of ileal villus enterocytes revealed chlamydiae in vacuoles or occasionally free in the cytoplasm. The results indicated that the swine chlamydial isolates used in this study are intestinal pathogens in gnotobiotic pigs.