In March 1998 we measured iron in the upper water column and conducted iron- and nutrient-enrichment bottle-incubation experiments in the open-ocean Subantarctic region southwest of Tasmania, Australia. In the Subtropical Convergence Zone (~42 ¿S, 142 ¿E), silicic acid concentrations were low (<1.5 &mgr;M) in the upper water column, whereas pronounced vertical gradients in dissolved iron concentration (0.12--0.84 nM) were observed, presumably reflecting the interleaving of Subtropical and Subantarctic waters, and mineral aerosol input. Results of a bottle-incubation experiment performed at this location indicate that phytoplankton growth rates were limited by iron deficiency within the iron-poor layer of the euphotic zone. In the Subantarctic water mass (~46.8 ¿S, 142 ¿E), low concentrations of dissolved iron (0.05--0.11 nM) and silicic acid (<1 &mgr;M) were measured throughout the upper water column, and our experimental results indicate that algal growth was limited by iron deficiency. These observations suggest that availability of dissolved iron is a primary factor limiting phytoplankton growth over much of the Subantarctic Southern Ocean in the late summer and autumn. ¿ 1999 American Geophysical Union