Bluetongue (BT) is an infectious, arthropodborne viral disease of domestic and wild ruminants caused by bluetongue virus (BTV), which is a double-stranded segmented RNA virus. Of the 26 confirmed BTV serotypes, 23 were reported in India based on the detection of antibodies or virus. In order to assess the prevalence of different serotypes in Andhra Pradesh, serum samples which were positive for BTV by group-specific antibody ELISA were subjected to type-specific neutralization of BTV serotypes 1, 2, 9, 10, 21 and 23. Of the 52 samples tested, 50.0, 44.23, 21.15, 26.92, 0, and 15.38 % neutralized BTV serotypes 1, 2, 9, 10, 21 and 23, respectively. However, 32.69 % of the ELISA positive sera could not neutralize any of these serotypes, indicating that there could be other serotype viruses (e.g., BTV-3 and -16) circulating in the State. This method can be used for surveillance of the circulating serotypes as well as for assessing the level of herd immunity, and assist in determining the vaccine strains to be used in multivalent vaccines.Keywords Bluetongue virus Á Serotype Á Surveillance Á Virus neutralization assay Á Epidemiology Bleuetongue (BT) is one of the major economically important livestock diseases in India. The disease is caused by bluetongue virus (BTV), a double-stranded segmented RNA virus belonging to genus Orbivirus of family Reoviridae. Clinical BT is observed only in sheep, and not in other domestic or wild animals. However, antibodies against BTV are frequently observed in cattle, buffaloes, goats and some wild ruminants, indicating asymptomatic infection in these species [21,23,25,27,36]. Twenty-six different serotypes of BTV are recognized worldwide and distinct topotypes defined by closely related sequences of each genome segment have been proposed [19]. In India, 23 serotypes of BTV have been reported, identified either by serology or virus isolation, and eight serotypes (BTV-1, -2, -3, -9, -10, -16, -21, and -23) have been isolated from different regions during the last decade [4,5,8,12,17,22,26,28,30,33,36,37].BT is endemic in India. Multiple serotypes of BTV commonly circulate in the same geographical area, and can be isolated from the same flock or even from the same animals. Further, recent reports indicate the incursion of Western topotypes of BTV into India [12,17,18,20,29,31]. Therefore, the identification of circulating serotypes of BTV is important. However, virus isolation followed by serotyping is time consuming. In addition, by the time the viruses are isolated and the serotype is determined, new serotypes may appear, resurface or dominate, making it difficult to select the serotypes for incorporation in vaccines. An inactivated polyvalant vaccine has been developed in India using five serotypes of BTV (BTV-1, -2, -10,