SummaryThe distribution of breeding colonies of Little PenguinsEudyptula minoron Phillip Island has diminished substantially in the past 90 years and breeding birds are now virtually restricted to the Summerland Peninsula. Declines were found in the numbers crossing Summerland Beach between 1977 and 1988, the number of birds breeding in the principal study area on the eastern boundary of the colony between 1968 and 1988 and the number of active burrows counted in the Parade area between 1980 and 1987. The extent of philopatry, emigration and immigration, changes in breeding success and survival of immatures and adults were investigated as possible factors reducing population size. The relative importance of adult and immature mortalities and breeding productivity to population levels was estimated and adult mortality was found to be far more significant. It is suggested that most immature and adult Little Penguins die at sea. Causes of mortality at sea and on land are reviewed together with population trends of this species elsewhere and compared to those of other species of temperate penguins.