The Cryptosporidium in the small intestine of domestic mice (Mus musculus) was initially described as Cryptosporidium parvum. Recent genetic and biologic characterization of Cryptosporidium isolates indicate that domestic mice are infected with several morphologically indistinguishable intestinal Cryptosporidium parasites with different host specificities, including C. parvum sensu stricto, mouse genotype I, and mouse genotype II. In this study, the morphological, biological, and genetic characteristics of the Cryptosporidium mouse genotype I are described. As a full re-description of C. parvum was made in 1985 for isolates from calves and humans and the name C. parvum has been widely used for the parasite that is infectious to both ruminants and humans, the mouse genotype I is named as Cryptosporidium tyzzeri. Oocysts of the new species (4.64±0.05 μm ×4.19±0.06 μm, with a mean shape index of 1.11±0.02; n=69) are slightly smaller than those of the re-described C. parvum. The prepatent period was six and seven days, and the patent period was 24-28 and 28-29 days in neonatal and adult mice, respectively. Oocysts were not infectious to lambs and calves. Light, transmission electron and scanning electron microscopy studies of the new species showed the presence of developmental stages in the microvillar brush border of the jejunum and ileum of experimentally infected mice, with the infection most intensive in the ileum. It had nucleotide sequences significantly different from C. parvum at the small subunit rRNA, 70 kDa heat shock protein, oocyst wall protein, actin, and the 60 kDa glycoprotein genes. Based on the morphological, genetic, and biological data and in compliance of established Cryptosporidium species naming criteria, this geographically widespread parasite is named as a new species in honor of Ernest Edward Tyzzer, who pioneered Cryptosporidium research.