Epidemiological surveillance of porcine rotavirus (PoRV) strains was carried out in Chiang Mai Province, Thailand, from 2002 to 2003, and eight rotavirus isolates could not be completely typed by PCR. Of these, six were G3 and one was G4 and displayed a P-nontypeable genotype, while another isolate was both G and P nontypeable. Analysis of a partial VP4 gene of all eight P-nontypeable strains revealed a high degree of amino acid sequence identities (94.7% to 100%), suggesting that they belonged to the same P genotype. Comparison of the amino acid sequences of two representative strains (namely, strains CMP178 and CMP213) with those of 27 other known P genotypes revealed a high degree of amino acid sequence identity with those of P Group A rotaviruses are the most important etiologic agents of severe diarrhea in young children and young animals worldwide. In developing countries, these severe diarrhea cases lead to an estimated 454,000 to 705,000 deaths annually among children under 5 years of age (32). Group A rotaviruses are members of the Reoviridae family with nonenveloped icosahedral particles. The mature virion is formed by three concentric layers of proteins that enclosed a genome of 11 segments of double-stranded RNA. Rotaviruses are classified according to the genetic and antigenic diversity of the two outer capsid proteins, VP4 and VP7. These proteins independently induce type-specific neutralizing antibodies and form the basis of the dual classification of group A rotaviruses into the P (protease sensitive) and the G (glycoprotein) serotypes, respectively (5, 17).Rotaviruses express extensive antigenic and genomic diversities. To date, at least 15 G genotypes and 26 P genotypes have been identified from humans and a variety of animal species (5,22,26,28,30,35,36). Most recently, several groups of investigators have proposed a novel genotype, P[27], which was isolated from diarrheic piglets (18,25,38). Generally, rotaviruses of the same G genotypes share at least 90 to 91% VP7 amino acid sequence identity (11,12,13,16). Rotavirus strains sharing Ն89% VP4 amino acid sequence identities are considered to belong to the same P genotype, while those sharing VP4 amino acid sequence identities of Ͻ89% belong to different genotypes (2, 5, 7). Moreover, the VP8* trypsin-cleavage product of VP4 coding for amino acids (aa) 13 to 250, including the greatest sequence divergent region (aa 71 to 204), correlates well with VP4 genotype specificity (20,21).Several epidemiological studies have demonstrated that among porcine rotaviruses (PoRVs), G3, G4, and G5, are the most common G genotypes and usually associate with the P[6] or P [7] genotype (5,23,39,40). In addition, other G and P types, such as G1, G2, G6, G8, G9, and G10 and P [13] (1,3,5,9,10,16,18,27,29,33,34,37,39,40,41). Accordingly, a comprehensive genotypic characterization of the rotavirus strains circulating in domestic animal populations, especially pigs, is important to define the extent of rotavirus diversity.Rotavirus strains bearing P[13] genotype spe...