Tetrahymena thermophila is a unicellular, ciliated freshwater protozoan. It is well suited for undergraduate research in that cells can be grown overnight to densities of 100,000 cells per ml or more. These cells exhibit a wide repertoire of behaviors that can serve as model systems for investigative analysis. Our laboratory has embarked upon a genetic analysis of the mating biology of Tetrahymena. Tetrahymena can be induced to mate in large synchronous cultures. Nuclear and chromosomal activities can be monitored with DAPI staining and fluorescence microscopy, or in real-time with Nomarski interference microscopy. Cytoskeletal rearrangements can be followed using indirect immuno-fluorescence and antisera to tubulin or other cytoskeletal proteins. One particularly striking feature of Tetrahymena mating is that at one particular time during conjugation, different nuclei within a common cytoplasm are simultaneously either (1) undergoing programmed nuclear elimination, (2) undergoing programmed gene rearrangement and chromosome amplification, or (3) remaining developmentally inert. Another feature of Tetrahymena biology is that we can functionally distinguish six (possibly seven) different "types" of nuclear division based upon mutant phenotypes.