Four double-stranded RNAs (dsRNAs), referred to as dsRNA 1 (3617 bp), dsRNA 2 (2794 bp), dsRNA 3 (2576 bp) and dsRNA 4 (1420 bp), were detected in the EGS 35-193 strain of Alternaria alternata at high concentration ( approximately 3 microg/g dried mycelium). This strain had an impaired growth phenotype. By exposing the strain to cycloheximide during hyphal tip isolation, we isolated strains which had normal mycelial growth and pigmentation, in which decreased levels of the dsRNAs were observed ( approximately 0.3 microg/g dried mycelium). These results indicate that this dsRNA mycovirus might be involved in modulating traits of its fungal host, A. alternata. The buoyant density of isometric virus particles (about 33 nm in diameter) containing these dsRNAs in CsCl was 1.35-1.40 g/cm(3) depending on the size of the packaged dsRNAs. The dsRNA 1 encodes a single open reading frame (3447 nt) containing the conserved motifs of viral RNA-dependent RNA polymerase (RdRp), which is related to the ORF encoded by dsRNA 1 of Aspergillus mycovirus 341. It is noteworthy that all of the coding strands of the four dsRNA genomes have 3'-poly (A) tails ranging from 33 to 50 nt in length. We named this novel dsRNA mycovirus in the EGS 35-193 strain A. alternata virus-1 (AaV-1).