Ethidium bromide staining of electrophoretically separated ssRNAs and dsRNAs as well as northern blot analyses with cDNA clones suggested that the genome of the Ahlum serotype of beet soil-borne virus (BSBV) consists of two major ssRNA species of approximately 3.6 and 3.2 kb, respectively, and possibly a minor ssRNA of approximately 6.0 kb. A few of our clones hybridized with both the 3.6-kb and the 3.2-kb RNAs, the majority of the clones, however, hybridized only with the 3.2-kb RNA. The 3.2-kb RNA is, therefore, apparently not a degradation product or a partially deleted form of the 3.6-kb RNA. None of our clones hybridized with the faint band(s) of the 6.0-kb ssRNA(s) which was produced by RNA extracts of some of our virus preparations. A fourth ssRNA of approximately 3.0 kb, which hybridized with the same clones as the 3.2-kb RNA, was found at relatively high concentrations in RNA extracts from purified virus, but not in total RNA extracts from leaves. Its origin is unknown. It is apparently not derived from the 3.2-kb RNA by the loss of a VPg or a poly(A) tail. Hybridization tests with 32P-labelled poly(dT) suggested that none of the RNAs of BSBV is polyadenylated. With respect to size and the lack of a poly(A) tail the RNAs of BSBV are more similar to those of definitive furoviruses than to those of beet necrotic yellow vein virus which is only a possible member of the furovirus group and has RNAs which readily hybridized with poly(dT).