Fil: Stupka, Juan. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas; Argentina.Fil: Degiuseppe, Juan Ignacio. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas. Servicio Hepatitis y Gastroenteritis; Argentina.Fil: Parra, Gabriel I. Universidad Nacional de Asunción. Instituto de Investigaciones en Ciencias de la Salud. Departamento de Biología Molecular; Paraguay.Fil: Argentinean National Rotavirus Surveillance Network; Argentina.Background: Group A rotaviruses are the leading cause of non-bacterial severe diarrhea disease in infants and young children. In humans, the most common genotypes are G1-G4 and G9. Recently, G12 strains have been sporadically reported in several countries, including Argentina, Brazil and Paraguay. Objectives: To analyze rotavirus strain diversity in Argentina during 2008-2009 and to describe the whole genome-based classification of emerging G12P[8] strains detected in our country. Study design: Rotavirus positive-samples (n=544) were collected from Argentinean children during 2008-2009, as a part of the National Surveillance Network for Viral Diarrheas. Specimens were genotyped by reverse transcription-polymerase chain reaction (RT-PCR) followed by nested-multiplex PCR. Sequencing of 11 genome segments was performed in 3 randomly selected G12P[8] strains. Results: G9P[8] was the most frequent strain in 2008, but in 2009 G3P[8] and G12P[8] were the most frequent strains in different geographical regions of the country. The novel emerging G12P[8] strains presented the following combination of genes: G12-P[8]-I1-R1-C1-M1-A1-N1-T1-E1-H1 (i.e. genotype1, Wa-like strains). The phylogenetic analysis of the VP7 gene of the G12P[8] strains grouped them within lineage III. Previously reported Argentinean G12P[9] strains presented genes from genotype 3 (AU-1-like strains) with a VP7 gene from lineage II. Conclusions: The emergence of G12P[8] rotaviruses was due to the introduction of a new strain, rather than to a reassortment of the G12P[9] strains previously circulating in our country. This study assesses the temporal and geographical changes in genotypes prevalence as well as the periodic emergence of unusual G genotypes.