Infectious myonecrosis virus (IMNV) infecting cultured Litopenaeus vannamei in Brazil is a double-stranded RNA virus that causes a slowly progressive disease with cumulative mortalities of up to 70%. The disease is currently diagnosed using a combination of gross signs (primarily skeletal tail muscle necrosis with white opaque discoloration), histopathology, and in situ hybridization with a digoxigenin-labeled gene probe. A rapid and sensitive method for definitive diagnosis of the disease was developed using reverse-transcriptase polymerase chain reaction (RT-PCR). Two primer sets were used to detect 328 and 139 bp amplicons in a nested RT-PCR assay. Using RNA extracted from purified virions, the first step reaction detected 100 copies of the IMNV viral genome whereas the nested step detected 10 copies. The primers were shown to be specific for IMNV and no amplicons were detected using RNA extracted from shrimp infected with other penaeid shrimp viruses (Taura syndrome virus [TSV], yellowhead virus [YHV], infectious hypodermal hematopoietic necrosis virus [IHHNV] and white spot syndrome virus [WSSV]).