The PAV, MAV, and RPV strains of barley yellow dwarf virus have been identified in west-central Morocco. The PAV strain was the most common of all (56%), followed by the MAV (35%), and RPV (9%) strains. The disease induced by the virus reached epiphytotic conditions in the 1986-87 growing season. The disease thrives in the spring months but the virus and aphid vectors, especially Rhopalosiphum padi, persist all year in the region. Although R. padi is the principal virus vector, other aphid species become active during spring months. The host range of BYDV in west-central Morocco encompasses a total of at least thirty-one grass species including those locally grown as well as those introduced for pasture improvement. Some of these species remain symptomless after inoculation with the virus;A total of ten grass aphid species in Morocco are now known to act as vectors of BYDV. Three of these were newly identified as vectors. Melanaphis donacis failed to transmit the PAV, MAV, and RPV strains of BYDV;Yield losses induced by inoculation of the bread wheat cultivars Nesma 149 and Saada with BYDV over two growing seasons reached as high as 39%, 24%, 38%, and 61% for head and thousand kernel weights, and total and grain yields, respectively. Respective losses due to natural infection were 11%, 11%, 14%, and 26%. Nesma 149 showed more tolerance to natural virus infection than Saada during the epiphytotic of 1986-87. The assessment of tolerance to BYDV demonstrated that 3 of 8 durum wheat, 4 of 19 bread wheat, 3 of 8 oat and none of 17 barley cultivars tested showed promising tolerance to the challenging PAV strain of BYDV;Antiserum prepared against a Moroccan PAV-like isolate of BYDV had a homologous titer of 1/3000 in indirect enzyme-linked immunosorbent assay. A virus yield of 1.45 mg/kg fresh weight of oat cultivar Clintland '64 was achieved by using a purification method comprising clarification with chloroform, precipitation with polyethylene glycol, and centrifugation through sucrose gradients.