Four splenectomized ponies were infected with Babesia equi. Two ponies were not treated, and two were treated with 5 mg of imidocarb/kg of body weight when 20% of their erythrocytes were parasitized. The nucleus of the parasite in erythrocytes from blood smears of nontreated ponies was roundish, deep red-staining, and usually composed two-thirds of the organism. The parasite in erythrocytes from blood smears of treated ponies was swollen and vacuolated except for an area just beneath the plasma membrane where the compressed nucleus was marginated, elongated, and clumpy. Parasites in erythrocytes from nontreated and treated ponies were examined by electron microscopy. Babesia equi in erythrocytes of nontreated ponies contained food vacuoles, a prominent nucleus, smooth and rough endoplasmic reticulums, and ribosomes. There were four stages of degeneration of B equi in erythrocytes of ponies treated with imidocarb. The first stage was dilatation of the nuclear cisterna and clumping of chromatin. The second stage included dissolution of the nuclear envelope and passage of nuclear matrix into the cytoplasm. The third stage consisted of vacuolation of the cytoplasm and compression of the chromatin to the periphery of the cell. In the fourth stage of degeneration, the organism consisted essentially of a vacuole, and chromatin was condensed into an irregular mass of clumped material.