DNA content, at different stages in the life cycle of the hemoprotozoan parasite Trypanosoma brucei, has been analysed with a fluorescence activated cell sorter. It was observed that the long slender bloodstream form stage and procyclic culture forms (analogous to the tsetse fly midgut stage) are dividing cell populations with cells in G1, S, G2 and mitosis. Short stumpy bloodstream form and metacyclic fly salivary gland form populations are composed of non-dividing parasites stabilized in G1 or G0 of the cell cycle. Haploids, possible sexual forms, were not detected. In response to transfer to a culture system which mimics the fly midgut, short stumpy bloodstream form parasites were readily able to initiate DNA synthesis and differentiate into dividing procyclic culture forms. This supports the suggested role of the short stumpy form as a transitional stage between the mammalian host and the tsetse fly vector. Analysis of early and late bloodstream populations of another salivarian trypanosome, Trypanosoma vivax, revealed a transition from dividing to stationary cell population similar to that observed with T. brucei. A hitherto unrecognized morphological form of T. vivax, analogous to the T. brucei short stumpy form, was detected. It is suggested that the long slender to short stumpy morphological transformation, long known in T. brucei, reflects a physiological transition from dividing to nondividing parasite relevant to the life cycle of all the salivarian trypanosomes.