Corn lethal necrosis (CLN) is caused by the synergistic interaction between maize chlorotic mottle machlomovirus (MCMV) and any potyvirus which infects cereals. Interactions between MCMV and wheat streak mosaic rymovirus (WSMV) in N28Ht corn produced MCMV concentrations that averaged 3.3- to 11.2-fold higher in doubly infected plants than the average concentrations in plants inoculated with MCMV. MCMV-negative sense RNA concentrations were similarly increased, and the ratio of full-length to subgenomic RNA was the same in singly and doubly infected plants. Contrary to most synergisms involving a potyvirus, WSMV infections were enhanced by the presence of MCMV. WSMV infection rates were higher when plants were coinoculated with MCMV, and the difference in infection rates was more pronounced at higher temperatures. Under conditions favorable for establishing high WSMV infection rates (cooler temperatures and high light intensity), WSMV concentrations in doubly infected plants averaged 2.1- to 3.1-fold higher than those in singly inoculated plants. Doubly inoculated plants with the lowest WSMV levels also had the lowest MCMV concentrations, but the concentrations of MCMV and WSMV in the most heavily infected plants did not directly correlate. These results suggest that there are genes in both MCMV and WSMV which directly or indirectly affect the replication and/or spread of the other virus in CLN.