Antarctic sea-ice concentration at Ocean Drilling Program Sites 1165 (64.380°S, 67.219°E) and 1166 (67.696°S, 74.787°E) was lower than today through much of the Pliocene. The low sea-ice concentration is evident from the proportion of the diatom Eucampia antarctica with intercalary valves (Eucampia index). This sea-ice proxy was calibrated by using modern diatom data obtained from core-top samples and winter sea-ice concentration data (September average through 1979–1987). The modern relationship is expressed as a binomial generalized linear model (modern sea-ice model). This model was applied to the Pliocene Eucampia index within a 95% tolerance interval (obtained from bootstrap estimates). The results indicate that reduced winter sea-ice concentrations persisted through much of the Pliocene and at times were 78% and 61% relatively less concentrated than today at Sites 1165 and 1166, respectively.