Black-tailed and white-tailed prairie dogs in north-central Colorado were compared in a study made from Jan. 1959 through Sept 1960. The black-tailed prairie dogs were well organized socially with males playing prominent roles. The organization of the white -tailed species consisted of loosely knit temporary family groups headed by the female parents. Both species used tactile, visual, and auditory communications. The species differed markedly in the type, number, and construction methods of the burrow entrances. The black-tailed prairie dogs altered their above-ground environment behaviorally, but the white-tailed did not. Both were diurnal, with 2 periods of maximum surface activity during the summer. Black-tailed prairie dogs generally confined their daily activities to definite boundaries; the white-tailed did not. Pups of both species spent considerable time in play. Pups of the black-tailed species displayed much care-soliciting behavior directed at the adults; pups of the white-tailed species did not. Black-tailed prairie dogs remained active above ground throughout the year, but with less total activity during winter than summer. White-tailed prairie dogs completely terminated surface activity during winter; adult males were the first to appear above ground in early spring and the first to become inactive in early summer; adult females appeared a little later and remained active slightly longer; pups remained active above ground until fall. Food habits varied with the time of year and the availability of vegetation. The black-tailed species ate mostly forbs and grasses, the white-tailed mostly grasses and sedges. The black-tailed prairie dogs consumed many roots; the white-tailed never ate roots. The density of burrow entrances and animals was greater for the black-tailed than for the white -tailed prairie dogs. The black-tailed species bred during late Feb; births occurred in late March and April, and the pups appeared above ground in early May. The white-tailed species bred in late March; births occurred in early May, and the pups appeared above ground in early June. There was no evidence to indicate yearlings were less successful breeders than adults with either species. There appeared to be no mortality between the time of conception and the initial appearance of the pups above ground for the black-tailed species, but almost 40% of the embryos were lost in this interval with the white-tailed species. Immigration and emigration played little part in the dynamics of the black-tailed species, but were more important for the white-tailed species. Badgers were the principal predators for both species and drowning contributed to mortality at both study sites. The black-tailed prairie dogs were longer than the white-tailed species, mostly because their tails were longer. Females of both species were similar in weight, but male white-tailed prairie dogs were heavier than the black-tailed of either sex.