SummaryA cDNA library was produced from the RNA of a Swedish MAV‐like isolate of barley yellow dwarf virus (BYDV). The procedure involved random priming and the ds cDNA was cloned into the EcoRl site of the plasmid pUC19. Among the clones obtained some hybridised specifically with MAV‐like isolates whereas others also hybridised with PAV‐like isolates. Only very weak hybridisation was observed with an RPV‐like isolate. An Australian cDNA clone, reported to be PAV‐specific (pBY82, Waterhouse, Gerlach&Miller, 1986), hybridised with Swedish MAV‐like but not with PAV‐like isolates. Probes prepared from the clones detected virus in plant extracts by dot‐blot hybridisation with sensitivity greater than that of ELISA. Virus was also readily detected in extracts of viru