Twelve cattle, three per group, were each inoculated orally with 100,000 Ostertagia ostertagi infective larvae when they were 2 months, 4.5 months, 12 months, and 24 months-old, respectively. Signs of clinical parasitism did not develop through 30 days when the infections were terminated with an anthelmintic. The cattle were each reinoculated with 500,000 O ostertagi larvae 6 days later. Groups I to III then developed anorexia and diarrhea after 3 weeks. One calf, inoculated at 2 months of age, died 27 days after reinoculation; two calves became emaciated. Group IV calves did not develop clinical signs of parasitic disease. All cattle were necropsied 42 days after reinoculation. Pathologic lesions, characteristic of type I ostertagiasis, were found in cattle of all groups; lesions were most severe in group I and least severe in group IV calves. Groups II to IV averaged 56,397 to 65,817 worms, whereas group I averaged only 29,916 worms. Group I calves expelled the worms during a period of profuse diarrhea. Cattle from 2 to 24 months old were readily infected and reinfected experimentally with O ostertagi, but the oldest cattle were most resistant to the pathogenic effects of the parasites.