Proof is given in this paper that dwarfing of wheat, which occurs in several regions of Czechoslovakia, is a virus infection. The virus was transmitted by means of naturally infected imagos of the leaf-hopperPsammotettix alienus DAHLB. to spring wheat and spring barley. It was also found that it is transmissible to a high degree by larvae grown from eggs which are then artificially infected at the source of infection. An attempt to transmit the virus by means of the aphidsBhopalosiphon oxycanthae (SCHRANK.) andSitobium granarium (KIRBY) MORDV. was negative. Nor was it possible to transmit it by soil, wheat grains or by mechanical inoculation with sap from diseased plants.The Russian mosaic virus of winter wheat would seem, in view of its vector, to be the most closely related to the wheat dwarf virus. It differs, however as regards the symptoms produced on the host cereals, by having a different vector and by the absence of inclusions in the cytoplasm of infected plants. These facts indicate that we are probably dealing with a new virus, which has not previously been described.