The present manuscript attempts a reassessment of the aboriginal population of Northwestern California, from the Oregon line to the Bay of San Francisco. There are no natural and fixed limits to the territory. Its outline serves merely the purposes of convenience. For this reason the individual units within the whole area are based, not upon natural ecological provinces such as mountain ranges, valleys, or river basins, but upon ethnic or "tribal" boundaries. Moreover, since there is no necessary interrelationship between the component parts, each is considered as a separate entity, and its population is computed separately. There is no final grand total to be added up, the significance of which transcends that of any of the constituents. Since the objective here is the calculation of pure numbers, it is irrelevant that the natural habitat, the mode of life, the reactions to environment of the various tribes and linguistic stocks vary enormously. Such a disregard for the basic principles of ethnography and human ecology will be tolerated only because the limitations of space and time demand that the fundamental question "What _was_ the population?" be answered before opening up the problem of _why_ the population was no greater or no less. We must know how many people there were before we can study their equilibrium with the physical or cultural environment. The outcome of this study is to augment markedly the previously estimated number of inhabitants in the region at hand, and, by implication, the number in the whole state. The magnitude of the aboriginal population has steadily diminished in our eyes for many years. I believe it was Powers who thought that the natives numbered as high as 750,000 or more. Merriam thought there were 260,000. Kroeber, in the Handbook of California Indians, (1925, p. 882) reduced it to 133,000. I myself in an earlier work (1943, pp. 161 _et seq._) reviewed the evidence and raised Kroeber's figure by no more than 10 per cent. It appears to me that the trend toward assessing the native population in continually diminishing terms is due to the operation of two factors.