Peste des petits ruminants (PPR) is an economically important viral disease of goats and sheep first described in west Africa in the 1940s. The virus has been circulating in parts of sub-Saharan Africa for several decades and in the Middle East and southern Asia since 1993, although the first description of the virus in India dates to 1987. To study the genetic relationship between isolates of distinct geographical origin, a selected region of the fusion (F) protein gene of the viruses was amplified using RT/PCR and the resulting DNA product sequenced for phylogenetic analysis. Viruses from 27 outbreaks in Asian and Middle Eastern countries, reported between 1993 and 2000, and two recent outbreaks from the African continent were compared with the prototype African strain. Of the four known lineages of PPR virus, lineage 1 and 2 viruses have been found exclusively in west Africa. Virus from an outbreak in Burkina Faso in 1999 fell into the lineage 1 group. Viruses of lineage 3 have been found in east Africa, where an outbreak in Ethiopia in 1996 was of this type, and also in Arabia and in southern India. However, there have been no further isolations of lineage 3 virus from India since the one reported in 1992 from Tamil Nadu. A virus of this lineage was found circulating in Yemen in 2001. In the past 8 years virus exclusively of the fourth lineage has spread across the Middle East and the Asian sub-continent, reaching east as far as Nepal and Bangladesh. This virus lineage was also reported from Kuwait in 1999. The geographical source of the new lineage 4 virus is unknown although it is most closely related to African lineage 1. The possibility that its earlier presence in northern India was masked by the circulation of Rinderpest virus, a related virus of cattle, is considered unlikely.