This study compared the course of infection by Ancylostoma caninum and Ancylostoma braziliense in mixed-breed dogs infected with L3 larvae. Dogs infected with A. caninum eliminated more eggs than did those infected with A. braziliense. A total of 38 % of A. caninum and 44 % of A. braziliense larvae were recovered as adult worms. There were no marked clinical abnormalities in dogs with either infection. A. caninum was associated with anemia and an increased number of circulating neutrophils, whereas infection with A. braziliense led to a decrease in the number of leukocytes. The humoral response against excreted and secreted antigens from adult worms was more sensitive and specific than the response induced with the crude antigen. No immune response was observed for either crude or excreted-secreted (ES) antigens from larvae of either species. A nonspecific response against the crude antigen of A. braziliense was found at 0 and 7 days postinfection and maintained throughout the infection period. However, antibody titers against ES antigens were elevated in A. caninum infection at patency and death, showing that this antigen has a higher specificity. The immune response elicited by infection with A. braziliense in dogs has not been described previously. No significant differences were observed in the infection processes of the two Ancylostoma species, except for the higher number of eggs eliminated from dogs infected with A. caninum, which may indicate a better evolutionary adaptation of the parasite to its host in comparison with A. braziliense.2035-01-01