The epidemiology of RPV- and PAV-like serotypes of barley yellow dwarf viruses (BYDVs) on winter barley in central Spain was studied by monitoring BYDV incidence, and the population dynamics of BYDV aphid vectors, in plots within a barley field. Virus incidence and virus vector populations were also monitored on alternative hosts, such as volunteer cereals and grass weeds in the autumn and winter, or grass weeds and irrigated maize in the summer. Two separate epidemiological systems, for RPV- and for PAV-like serotypes of BYDV, appear to be superimposed. For the RPV-like BYDV, local sources of virus and virus vectors are important and there is a high incidence of BYDV infection of barley during winter. For the PAV-like BYDV system, long-distance sources for virus and vectors are most important and cause infection of barley in the spring. Irrigated maize does not appear to be an important vector and virus reservoir for the infection of winter barley, but can be infected by aphids migrating from barley in early summer. © 1993.