During field studies on the ecology of arboviruses in Italy, strains of a new virus were isolated from sand flies, mostly females, of a pure colony of Phlebotomus perniciosus collected in a central Italian region (Tuscany). The prototype was named Toscana (TOS) virus, assigned to the Phlebotomus fever serogroup of arboviruses (family Bunyaviridae, genus Phlebovirus) and registered in 1980 in the International Catalogue of Arboviruses. Neutralizing antibodies to TOS virus were detected in human sera. Serologic surveys were carried out to estimate the natural distribution of TOS virus in Italy and it was shown that antibodies to this virus could be detected in humans mainly of the Tuscany region. In particular, a high infection rate (24.8%) was observed among residents of the province of Florence. Furthermore, several patients with a diagnosis of aseptic meningitis presented a serologic response which reflected a recent infection with TOS virus in the Tuscany and Marche regions. Consequently, between 1980 and 1986 studies were conducted at selected sites of the Tuscany region to determine the possible occurrence of vectors and presence of foci of TOS virus and to assess its public health importance. As a result of these investigations, several virus strains were isolated from pools of wild caught Ph. perniciosus and Ph. perfiliewi. Of the virus isolates obtained, 37 were identified as TOS virus and 47 as a new serotype, member of the Phlebotomus fever group. For this virus we suggested the name of Arbia (ARB) virus, Arbia being the river flowing across both Florence and Siena provinces from which the isolates originated.(ABSTRACT TRUNCATED AT 250 WORDS)