Brown rot of stone fruit caused by Monilinia fructicola (G. Wint) Honey is one of the most common fungal diseases in California. In this study, two laboratory-induced iprodione-resistant (LIR) mutants of M. fructicola were characterized by osmotic sensitivity, virulence on prune and sequence of the two-component histidine kinase gene (Mfos-1). The LIR mutants showed more sensitivity to osmotic stress and lower virulence on prune than their wild-type parent. Analysis of deduced amino acid of Mfos-1 showed that this protein exhibited all the characteristic features of the two-component histidine kinase genes, including osmotic sensing domain, six 90-amino-acid repeat motifs (coiled coil region) and kinase core and response regulator domains. Comparison of DNA sequences of the Mfos-1 from LIR mutants and the wild-type sensitive (S) isolate showed that LIR mutants had single point mutations in the coiled coil region of Mfos-1.