THE El Nino/Southern Oscillation (ENSO) is a quasiperiodic variation in climate which arises from a complex interaction between the tropical Pacific Ocean and the atmosphere(1). ENSO events, which occur every two to seven years, are the largest source of interannual variability of temperature and precipitation on a global scale, although their effects are most profound in the tropics(1). Observations of sea-ice margins have been used to monitor global climate changes on timescales of greater than a decade(2), and there is some evidence for interannual variations in records of sea-ice cover(3). But short-term changes in sea-ice cover are masked by pronounced seasonal variations, making it difficult to correlate them with specific climate phenomena. Using a multiple-window harmonic analysis technique(4-8), I show here that time series of sea-ice cover from the Arctic and Antarctic contain statistically significant quasi-biennial and quasi-quadrennial periodicities that agree well with variations in the ENSO index. The response of sea ice to these two frequency components varies greatly for different regions.