The distribution of orthosilicic acid (Si(OH)4), lithogenic silica (LSi), biogenic silica (BSi), and biogenic silica production rates (&rgr;Si) have been investigated on a transect encompassing the Subantarctic Zone (SAZ) from the Subtropical Zone (STZ) to the northern Polar Frontal Zone (PFZ) in the Australian sector of the Southern Ocean at the end of summer 1998. Si(OH)4 distribution was characterized by a north-south gradient (surface values: 2 μM in the northern PFZ). The SAZ and STZ exhibited very low BSi values, while a deep (75--150 m) BSi maximum (up to 2.34 μmol Si L-1) characterized the PFZ. BSi growth rates were higher in the SAZ and STZ (up to 0.46 doubling d-1) as compared to the PFZ (0.01--0.10 doubling d-1) where the deep maximum probably resulted from phytodetritus accumulation at the end of the productive season. A very high KS value (30.7 μM Si(OH)4) was estimated in the PFZ surface waters, and it is concluded that multiple limitations including light, iron, and silicic acid availability act on the different subsystems. The study period is characteristic of the last stage of the seasonal cycle especially in the PFZ where &rgr;Si values were very low (mean value: 2.58 mmol Si m-2 d-1) as compared to previous studies. In the SAZ and the STZ respective &rgr;Si mean values were 1.07 and 0.60 mmol Si m-2 d-1, and the siliceous component does not appear to dominate the phytoplankton community. ¿ 2001 American Geophysical Union