Carcasses of 11 medium-sized species of mammals along two-lane highways in northern Central Valley, California, were counted between 1997 and 2007 to obtain an indirect measure of changes in mammalian populations over time. Over this period, 1.23 mammalian carcasses were recorded per 100 km, lower than that reported in several other parts of North America. Each species was classified as human-sensitive, suburban-adaptable, or human-adapted according to the use of the land in which it was principally located. In contrast to simple predictions that populations of more human-sensitive species would be in decline and human-associated species would be increasing, indices of populations of all species showed little change once volume of traffic had been taken into account. Therefore, populations of most of the common mammals in the study area appear relatively stable at present. Annual fluctuations in populations of one species did not mirror those of other species of mammals suggesting that each species is a poor indicator of changes in populations of the other sympatric species of mammals.