At intervals during the past few years Prof. G. E. MlacGinitie, of the Kerckhoff Marine Laboratory, Corona Del Mar, California, has sent us specimens of Polychaeta collected by him in the vicinity of his laboratory, or at points not more than a few miles distant. This is the source of most of the material described in this paper. In addition some of the specimens were collected by Dr. MacGinitie at other localities in Southern California. Yet others by Dr. W. G. Hewatt, of the Texas Christian University, at Santa Cruz Island during the summer of 1939 and kindly submitted by him to us for report. Forms taken both intertidally and by dredge are included. Notes indicating the depth of dredging operations did not always accompany the specimens, but the depth is given in the following pages when known and the locality at which each species was collected is stated unless this was Corona Del Mar or the vicinity If no statement to the contrary appears the species under discussion in the notes which follow was collected intertidally. <br /><br />The collection comprises 154 species. Thanks to the pioneer work of Johnson, Moore and Chamberlin and, more recently, to that of Treadwell, Hartman and others, the polychaete fauna of the Californian coast is fairly well known. Nevertheless the present collection contains 45 species, or varieties, new to the coast, of which 9 are new to science. Types of the new species and varieties are in the authors' collection. <br /><br />The following is a complete list of the species represented; those we believe to be new to California are indicated by an asterisk.