SCRIPPSIELLA sweeneyi and FRAGILIIDIUM heterolobum, new genera and species of Peridiniacea from the Pacific Ocean near San Diego(California) are described. A detailed description of the plate pattern, including cingulum and sulcus, and some information on nuclei, flagella, chromoplasts, movements and division are also given. Both organisms, kept in culture at Scripps Institution of Oceanography, showed some variation in plate pattern.