Fungal isolates belonging to four Rhizopus species were screened for the presence of double-stranded RNA (dsRNA) molecules. Five (two R. stolonifer, two R. microsporus, and one R. oryzae) of the 27 isolates examined harboured such genetic elements. Electrophoresis of the nucleic acids revealed five RNA patterns, with 1-5 discrete dsRNA bands. The molecular sizes corresponding to these bands were 2.2-14.8 kb. Gel electrophoresis of purified virus-like particles (VLPs) indicated only one capsid of similar size in all virus-harbouring strains; when investigated by electron microscopy, they were found to be polyhedral VLPs 40 nm in diameter. In one of the R. microsporus isolates an uncapsidated large dsRNA segment (14.8 kb) was observed. No phenotypic differences were observed between uninfected and virus-harbouring Rhizopus isolates.