The author, who is one of the world’s most distinguished authorities on aphids, presents the results of extensive collecting activity while sojourning in California during the winter of 1963-1964 and the following spring. The research recorded in the present paper is destined to be of great value to entomologists everywhere. Complete descriptions are given of many aphid species not previously known to occur in California, together with a number of keys. In addition, several other species in this category are noted, and certain earlier misidentifications are corrected. (See pages 622 and 623 for a classified, alphabetical index to the many genera and species treated.) Mr. Hille Ris Lambers’ collecting trips took him through the University of California campus and other parts of Berkeley; into the San Francisco sand dunes; and to various areas in the counties of Contra Costa, Marin, Monterey, Napa, Riverside, San Diego, Santa Clara, and Santa Cruz. He was accompanied and guided on these excursions by faculty members from three campuses of the University, who were of much assistance in their respective territories; in some cases, submitting valuable mounted material for study.