>We have obtained several autonomously replicating plasmids in procyclic Trypanosoma brucei. Two of these have been analyzed in detail. Both exist as monomeric single-copy episomes, which nevertheless demonstrate significant stability. These episomes transform procyclics at frequencies that are at least 60-fold higher than those achieved earlier with non- replicating molecules. A modified version of one of these episomes is able to replicate autonomously in bloodstream form T.brucei but it is unstable in this developmental stage of the organism. These episomes will prove useful as shuttle vectors, in functional complementation studies, in the construction of promoter-traps for the isolation of transcriptional promoters in T.brucei, and importantly, as models for the analysis of DNA replication in this ancient eukaryotic lineage.