Diatomaceous earth samples, known to contain silicoflagellates, were collected (from previously described localities in California) during the summer of 1962, and processed in the laboratory. From the 123 samples processed, silicoflagellates were found in 113 of these. A total of 210 slides were made from which over 190,000 separate microscopic fields were examined, and data collected on 43,000 silicoflagellates. Physical factors that influence silicoflagellate production were discussed and a silica cycle was presented. Known ecology of these organisms was utilized ·co infer paleoecologic conclusions. The major findings of this study are summarized as follows: (1) Silicoflagellates have value as stratigraphic markers. (2) Silicoflagellates are exceedingly well adapted to their environment. (3) Silioflagellates occur in sediments as a thanatocoenosis and any paleoecologic information drawn from their presence should be viewed with caution. (4) Silicoflagellates are excellent indicators of the temperature of the waters that they inhabit. (5) Variation in range of these organisms is an adaptation to their environment and appears to be a reliable temperature indicator. This study provides a more complete list of synonyms under the species described; adds information to the biostratigraphy of the Class from California sediments; adds size and population data to each of the species discussed; develops a systematic key for generic and specific identification and emends species descriptions where necessary.