UmV is a double-stranded RNA (dsRNA) virus of the corn fungal pathogen Ustilago maydis. UmV has no infectious cycle. Some UmV subtypes have viral dsRNAs encoding secreted toxins that kill sensitive cells of the same species and related species. There are three viral subtypes, P1, P4 and P6, which differ in the specificity of their secreted killer toxins. Each has three size classes of dsRNA: H (heavy), M (medium) and L (light). The L segments of UmV are unique in being derived from one end of the larger M segments. We have sequenced P1 L and placed it at the 3' end of the P1 M1 plus strand. In their overlapping regions, these dsRNAs are identical in sequence. In vitro translation of P1 M1 results in a peptide whose size is consistent with its being encoded by the non-L region of M1. P1 L is a very small dsRNA of 355 bp. It has no long open reading frames and produces no detectable in vitro translation product. The sequence of P1 L suggests that it is derived by a process unique among dsRNA viruses: replication and packaging of the 3' end fragment of a processed mRNA.