A network of membrane-bound organelles, interconnected by dynamic vesicular traffic, is a defining hallmark of eukaryotes. Our laboratory studies membrane traffic in the ciliate Tetrahymena thermophila. Our interest in these cells stems from the fact that Tetrahymena is unicellular and offers a host of experimental advantages; the individual cells are remarkably complex and include features that are usually associated with animal cells but absent in fungi. For example, ciliates have a prominent pathway for regulated secretion of polypeptides from specialized vesicles resembling dense core granules. This is in spite of the fact that ciliates are far more distantly related to animals than are most organisms, including yeast, that are commonly studied by cell biologists. We use genetic, cell biological, informatics and biochemical approaches to dissect this and other pathways of membrane trafficking in these remarkable cells.