Intensive studies were made during the past three years on the effect of barley stripe-mosaic (false stripe) on wheat and barley. The identity of this disease with the disease described and named by I. L. Conners in 1925 and reported by H. H. McKinney to be of virus origin in 1951 was established. Under both greenhouse and field conditions, infection caused chlorosis and necrosis accompanied by a marked loss of vigor. Large and statistically significant reductions in the height of crop, the yield, the kernel weight, and the grade of wheat resulted from the early inoculation of field plots, but not from late inoculation. The yield of wheat was reduced by 75% and of barley by 64%. The possibility that this disease has been a factor in the unthriftiness of certain fields of wheat is discussed.