Artículo donde se describe el primer caso de Síndrome del alevín de la trucha arcoíris (RTFS) en México; se detallan trabajos de caracterización.In 2015, rainbow trout (weighing 4 ± 0.5 g) mortalities occurred in an aquaculture facility near from “Nevado de Toluca” National Park in Mexico. Affected fish showed exophthalmia, body darkening, abdominal distension and pale gills. Two spleen isolates were recovered in pure culture and identified by biochemically tests and PCR assays as belonging to Flavobacterium psychrophilum. Virulence studies suggest that Mexican isolates have ability to cause disease and could be considered as a potential pathogen for rainbow trout aquaculture. To our knowledge, this is the first identification and characterization of F. psychrophilum as the agent of severe outbreak in Mexican rainbow trout aquaculture.Proyecto UAEM: 3675/2014/CID Diagnóstico y caracterización molecular de enfermedades emergentes de peces y por la Comisión Nacional de Investigación Científica y Tecnológica de Chile: CONICYT/FONDAP15110027; FONDECYT 1150695