The infectious pancreatic necrosis virus (IPNV) affects several species of freshwater and marine fish. In Mexico, IPNV has an important impact on farming of rainbow trout Oncorhynchus mykiss however, IPNV distribution in Mexico is unclear. The diagnosis of IPNV is laborious usually it is based on isolation tests in cell culture followed by immunological identification using techniques of serum neutralization, immunofluorescence, or enzyme-linked immunosorbent assay. It has recently been demonstrated that reverse transcriptase polymerase chain reaction (RT-PCR) is an adequate method for the detection of aquatic birnaviruses. However, its diagnostic use is still limited because very low titers of viable virus cannot be easily detected. In this study, a combination of short-time cell culture and multiplex RT-PCR was established for the diagnosis of IPNV in rainbow trout obtained from farms in the state of Mexico. Three primer sets were used in a single reaction in the multiplex RT-PCR to increase the probability of identifying all serotypes of IPNV serogroup A as well as to help prevent a false-negative result. This approach was able to identify samples with an IPNV concentration of just 0.01 tissue culture infective dose with 50% endpoint (TCID50)/mL, and it identified more infected fish than RT-PCR alone or first-passage cell culture alone. Moreover, this technique made the same identifications as second-passage cell culture but in approximately 30% of the time needed for second-passage cell culture. Consequently, the time and cost efficiency of IPNV diagnosis were greatly reduced.J Aquat Anim Health. 2009 Sep21(3):167-72.Barrera-Mejía M, Simón-Martínez J, Salgado-Miranda C, Vega F, Ortega C, Aragón A.Centro de Investigación y Estudios Avanzados en Salud Animal, Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma del Estado de México, Carretera de Cuota Toluca-Atlacomulco, Kilómetro 15.5, Código Postal 50200, Toluca, Estado de México.