Nanoviruses are multipartite single-stranded DNA (ssDNA) plant viruses that cause important diseases of leguminous crops and banana. Little has been known about the variability and molecular evolution of these viruses. Here we report on the variability of faba bean necrotic stunt virus (FBNSV), a nanovirus from Ethiopia. We found mutation frequencies of 7.52 x 10(-4) substitutions per nucleotide in a field population of the virus and 5.07 x 10(-4) substitutions per nucleotide in a laboratory-maintained population derived thereof. Based on virus propagation for a period of more than 2 years, we determined a nucleotide substitution rate of 1.78 x 10(-3) substitutions per nucleotide per year. This high molecular evolution rate places FBNSV, as a representative of the family Nanoviridae, among the fastest-evolving ssDNA viruses infecting plants or vertebrates.