The morphology of the peripheral blood leukocytes of the tammar wallaby, macropus eugenii, was investigated using both light and electron microscopy. In a study of 44 captive tammar wallabies, the five main families of mammalian leukocytes were identified and found to be similar to those of eutherian mammals and other marsupial species studied to date. Lymphocytes ranging in size from 5.5 to 17.5 m were characterised by high nuclear to cytoplasmic ratio whilst monocytes were generally larger than lymphocytes at 12.4-23.4 m and contained a single kidney shaped nucleus. Of the granulocytic populations only neutrophils and eosinophils were frequently observed and contained characteristic cytoplasmic granules. Whilst there have been a number of studies of the morphology of peripheral blood leucocytes in marsupials, a detailed description of the cells of the model marsupial, the tammar wallaby, has, to date, been lacking. This paper presents the results of the first comprehensive study of the morphology and ultrastructure of the peripheral blood cell populations of the tammar wallaby and reports that these cells are similar, though not identical, to those of eutherian mammals.