Most reported U.S. zoonotic cases of babesiosis have occurred in the Northeast and been caused by Babesia microti. In Washington State, three cases of babesiosis have been reported previously, which were caused by WA1 (for "Washington 1")-type parasites. We investigated a case of babesiosis in Washington in an 82-year-old man whose spleen had been removed and whose parasitemia level was 41.4%. The complete 18S ribosomal RNA gene of the parasite was amplified from specimens of his whole blood by polymerase chain reaction. Phylogenetic analysis showed the parasite is most closely related, but not identical, to B. divergens (similarity score, 99.5%), a bovine parasite in Europe. By indirect fluorescent-antibody testing, his serum reacted to B. divergens but not to B. microti or WA1 antigens. This case demonstrates that babesiosis can be caused by novel parasites detectable by manual examination of blood smears but not by serologic or molecular testing for B. microti or WA1-type parasites.