ABSTRACT Sugarcane striate mosaic (ScSM)-affected sugarcane leaves contain a disease-associated 9-kilobase (kb) double-stranded RNA (dsRNA), usually together with 6- and 2.6-kb dsRNAs. The purified 9-kb dsRNA was amplified by the randomly primed polymerase chain reaction (PCR) and cloned. The nucleotide sequences of three separate regions, representing about 2.55 kb (28%) of the dsRNA sequence, were found to have significant similarities to viruses in the genera Capillo-, Carla-, Fovea-, Potex-, Poty-, Tricho-, and Tymovirus. Greatest overall similarity was found to apple stem pitting virus, with less similarity to blueberry scorch virus and potato virus M. A standard virus purification procedure was used to identify slightly flexuous filamentous particles that copurified with the disease-associated RNA. Particle modal lengths were approximately 950 and 1,900 nm with a diameter of 15 nm. Preparations contained a 51-kDa putative capsid protein and a 9-kb single-stranded RNA with a probable 3' polyadenylate tract. These ScSM-associated virus particles differ physically from viruses in existing genera because of their relative rigidity, length, and putative coat protein size. Reverse-transcription PCR with a primer pair designed from the sequenced segments amplified a 820-base pair fragment from ScSM-affected but not healthy sugarcane plants.