Abstract The major groups of siliceous microfossils found in oceanic surficial bottom sediments of the Gulf of Mexico include: diatoms, radiolaria, sponge spicules and phytoliths. Silico‐flagellates, pyrrophytic endoskeletons (Actiniscus), resting spores of diatoms and fresh‐water Chrysophyte cysts are present as minor components in the sediments. Throughout most of the Gulf the siliceous forms are subordinate in abundance to calcareous forms (coccolithophytes and foramini fera). Diatoms, radiolaria and sponge spicules display high relative frequencies in the southeastern Gulf, especially off the West Florida and Campeche carbonate platforms. All three groups show low relative frequencies in the northwestern part of the basin and have a patchy distribution in areas of the eastern Gulf on a line from the mouth of the Mississippi River southeastward toward Cuba. This pattern reflects water mass productivity, terrigenous input and silica dissolution in the Gulf. Phytoliths display higher relative frequencies...