A subsurface chlorophyll maximum (SCM), composed principally of large diatoms, has been consistently observed in summer and autumn between 53¿ and 58 ¿S along 140 ¿E on Australian World Ocean Circulation Experiment and Joint Global Ocean Flux Study cruises from 1994 to 1998. This region lies in a zone of weak northward flow, between two branches of the Polar Front (hence the Interpolar Frontal Zone (IPFZ)). In the IPFZ, mixed layer nitrate concentrations are high (>24 μM) year-round, while mixed layer silicic acid is intermediate (about 15 μM) in winter but is depleted to 2 μM or less in late summer. Dissolved Fe concentrations, only available for summer, are low (<0.2 nM). Mixed layer chlorophyll concentrations are generally <0.3 μg L-1 and surface waters thus fit the high-nitrate low-chlorophyll (HNLC) definition. In spring and early summer, the SCM is relatively shallow (about 60 m), intense (up to 1.5 μg chl a L-1), and contributes 30--50% of column production. By March the SCM is deep (100 m or greater), less intense (about 0.5 μg chl a L-1), and contributes at most 20% of column production. The existence of a SCM in a HNLC region is surprising, and we consider a number of possible explanations. The SCM may be partly explained by changes in C:chl a ratios, but phytoplankton species composition in the SCM also differs from that in the mixed layer. Sinking of diatoms, mediated by Fe and/or silicic acid availability, appears to play an important role in the formation and maintenance of the SCM. ¿ 2001 American Geophysical Union