The bodonids and cryptobiids represent an early diverged sister group to the trypanosomatids among the kinetoplastid protozoa. The trypanosome type of uridine insertion-deletion RNA editing was found to occur in the cryptobiid fish parasite Trypanoplasma borreli. A pan-edited ribosomal protein, S12, and a novel 3'-and 5'-edited cytochrome b, in addition to an unedited cytochrome oxidase III gene and an apparently unedited 12S rRNA gene, were found in a 6-kb fragment of the 80-to 90-kb mitochondrial genome. The gene order differs from that in trypanosomatids, as does the organization of putative guide RNA genes; guide RNA-like molecules are transcribed from tandemly repeated 1-kb sequences organized in 200-and 170-kb molecules instead of minicircles. The presence of pan-editing in this lineage is consistent with an ancient evolutionary origin of this process.The evolutionary origin of the uridine (U) insertion-deletion type of RNA editing which occurs in the mitochondrion of the trypanosomatid kinetoplastid protozoa is an intriguing problem. Within the kinetoplastid protozoa there are two suborders, the Trypanosomatina and the Bodonina, as determined by morphological characters and life cycles. The trypanosomatids consist of approximately 8 to 10 obligately parasitic genera in a single family, and the bodonids consist of two major families containing both free-living and parasitic organisms (the Bodonidae and the Cryptobiidae) (30). RNA editing in several trypanosomatids, including the digenetic genera Trypanosoma and Leishmania and the monogenetic genera Crithidia, Blastocrithidia, and Herpetomonas, has been investigated (7,26,28). It has been shown previously that extensive editing or pan-editing, which is mediated by multiple overlapping guide RNAs (gRNAs), represents a primitive character state within the trypanosomatid lineage and that 5' editing and even loss of editing represent derived traits which possibly arose on account of replacement of pan-edited original versions of the genes, with partially or fully edited versions derived from mRNAs (11)(12)(13)(14). Little is known about mitochondrial DNA in the bodonids and cryptobiids, which is also termed kinetoplast DNA since it is present as a large DNAcontaining structure situated within the single mitochondrion (Fig. 1). In the free-living bodonid Bodo caudatus, there is no network of catenated minicircles and maxicircles such as that which exists in the trypanosomatids; instead, a variety of large circular molecules with heterogeneous sizes is found (8). In this paper, we show the occurrence of the uridine insertiondeletion type of RNA editing in the cryptobiid Trypanoplasma borreli, with some novel features, and we show that small gRNA-like transcripts are encoded in tandemly repeated 1-kb