Living specimens of Distephanus speculum (Silicoflagellata) were collected in waters of <150 m depth in the Pacific Sector of the Southern Ocean (57 degrees 31.6'S, 150 degrees 02.9'E), South of Tasman Island, and a biometric study has been conducted. The skeleton of D. speculum has a simple morphology, which can be readily expressed quantitatively. Using a new methodology, repeated measurements were obtained and converted to numerical indices. The circumference of the basal rings was the most useful character for subdivision into groups, and using this parameter we recognized two populations in the 620 studied specimens: (1) 500 individuals were placed in Group A, with a basal ring circumference of 53 to 67 pm, and a difference in length of spine axes from 2 to 6 pm; and (2) 120 individuals were placed in Group B (basal ring circumference 80 to 105 pm, difference in length of spine axes 3 to 6 pm). The circumference of the basal ring in Group B is approximately 10 pm smaller than in Distephanus speculum giganteus [Bukry, D.. 1979. Comments on opal phytoliths and stratigraphy of Neogene Silicoflagellates and Coccoliths at Deep Sea Drilling Project Site 397 off Northwest Africa. Initial report of the DSDP, Washington, 49:977-1009.], and Distephanus boliviensis [Bukry, D., 1976a. Silico-flagellate and coccolith stratigraphy, Norwegian-Greenland Sea. Initial Report of DSDP LEG 38. Initial report of the DSDP, Washington, 38:843-855.; Bukry, D., 1976b. Silico-flagellate and coccolith stratigraphy, southeastern Pacific Ocean, Deep Sea Drilling Project Leg 34. Initial report of the DSDP, Washington, 34:715-735]. (C) 2009 Elsevier B.V. All rights reserved.