SummaryBetween 29 and 47% of the interannual variability in the weight and breeding success of the Little PenguinEudyptula minoron Phillip Island, south-eastern Australia, was attributed, by correlation analysis, to monthly and interannual decreases in the east-west sea temperature gradient across Bass Strait. A station to the east, influenced by the East Australian Current, was generally about 2°C warmer than a station near the colony but these temperatures decreased, especially to the east, with the arrival of west winds. The correlations between physical and biological variables were consistent with the hypothesis that winds from the west bring cooler waters into and across Bass Strait, and that those cooler waters have slightly higher concentrations of nutrients or chlorophyll or more of the fish on which Little Penguins feed. The birds' biological response was measurable on a time scale of months: a decreased sea temperature gradient was associated with increased weights of adults four months later and a July-August decrease in sea temperature gradient was associated with an early start to egg laying in September-October.