Abstract. The morphology and taxonomy of the fossil diatom resting spore morpho-genera Xanthioisthmus Suto gen. nov. and Quadrocistella Suto gen. nov. are described. The two new genera are probably fossil resting spores of the marine diatom genus Chaetoceros. They were studied by examining samples from DSDP Sites 436, 438 (northwest Pacific) and 338 (Norwegian Sea), and the Newport Beach Section (California). The genus Xanthioisthmus is characterized by an elongate valve composed of two flat circles joined together by a hyaline broad isthmus and includes five species: X. biscoctiformis (Forti) Suto comb. nov., X. specticularis (Hanna) Suto comb. nov., X. panduraeformis (Pantocsek) Suto comb. nov., X. praemaculata sp. nov. and X. maculata (Hanna) Suto comb. nov. The genus Quadrocistella differs from Xanthioisthmus by its elongate and rectangular valve and bears five new species: Q. rectagonuma sp. nov., Q. tubera sp. nov., Q. paliesa sp. nov., Q. montana sp. nov. and Q. palmesa sp. nov.