Studies were conducted on the subcutaneous and intraperitoneal susceptibility to Brucella abortus, B. suis, B. melitensis, and B. neotomae of laboratory mice, guinea pigs, and rabbits and of 12 species of wildlife. These wild mammals included: deer mice (Peromyscus maniculatus), pinyon mice (Peromyscus truei), montane meadow mice (Microtus montanus), desert wood rats (Neotoma lepida), Ord and chisel-toothed kangaroo rats (Dipodomys ordii and Dipodomys microps), white-tailed antelope squirrels (Ammospermophilus leucurus), and black-tailed jack rabbits (Lepus californicus). The mice were generally found to be relatively susceptible to these bacteria, while the rats, lagomorphs, and the squirrels were more resistant. The persistence of serum agglutinins and viable organisms in the tissues of certain of these animals was determined up to 24 months after infection. All four bacterial species caused chronic infection in the animals, but no evidence was obtained to show that they pass the organisms in their excreta or other body wastes. Bacteremia was found to occur regularly in many of the animals during the early stages of infection, and the possibility of transmission by ectoparasites was noted. Mule deer (Odocoileus hemionus) and sheep were found to be relatively resistant to locally isolated strains of B. neotomae.