Equine rhinovirus serotype 3 isolate P313/75 was assigned, with an unclassified genus status, to the family PICORNAVIRIDAE: The sequence from the 5\' poly(C) tract to the 3\' poly(A) tract of P313/75 was determined. The sequence is 8821 bases in length and contains a potential open reading frame for a polyprotein of 2583 amino acids. Sequence comparison and phylogenic analysis suggest that P313/75 is most closely related to the prototype equine rhinitis B virus (ERBV) strain P1436/71, formerly named equine rhinovirus type 2. A high degree of sequence similarity was found in the P2 and P3 regions of the two genomes. However, the deduced amino acid sequences of the P1 region of P313/75 and ERBV strain P1436/71 contained significant differences, which presumably account for the serological segregation of the two viruses. It is suggested that P313/75 can be classified as a new serotype of the genus Erbovirus, tentatively named ERBV2. Seroepidemiological data indicate that ERBV2 infection of horses may be common (24%) in Australia.