The presence of virus-derived RNA was investigated in 38 axenically growing Giardia isolates from different geographic areas. The RNA virus was demonstrated in Giardia strains from humans in the U.S.A., England and the majority of strains from Poland. Two strains isolated respectively from a cat and a cavia also contained it. Giardia strains from humans in Belgium and Israel did not contain this RNA virus. Transfection of the RNA virus was accomplished from English and Polish strains, as well as from the cat isolate to isolates lacking it. Differences were observed both in sensitivity of Giardia strains to transfection and in infectivity of the RNA virus from different Giardia strains. Transfection could be carried out with sonicated Giardia extract as well as with filter sterilized medium in which Giardia strains containing RNA virus had grown. The RNA virus did not replicate in Giardia-free medium. No correlation could be demonstrated between the presence of the RNA virus in Giardia isolates and their in vitro resistance to some antiprotozoal drugs, nor with the fact that the strain originated from symptomatic or asymptomatic carriers. The presence of the RNA virus in Giardia trophozoites did not influence the isoenzyme patterns or restriction endonuclease patterns of repetitive DNA. A correlation may exist with the length of time since the isolation in axenic culture of the strain.