Symptoms of potato virus Y (PVY) have been observed in each of the past several years in burley tobacco growing in the Granada zone of Spain. Investigations were implemented to (1) confirm the presence of PVY and identify strains; (2) identify other significant viruses; (3) define incidence of infections; and (4) determine the effects on yield, quality and leaf chemistry associated with the virus infections. Five distinct sets of symptoms, involving stuting, vein _ banding, necrosis, and mottling, were observed. Serological and symptomatic observations revealed the presence of cucumber mosaic virus (CMV) alone and in combinations with necrotic and mild strains of PVY. The two strains of PVY were observed to collectively infect more than 90% of the burley tobacco in Granada while CMV was observed to occur in 25% of the population. Natural infections of CMV alone and necrotic PVY alone reduced yields 50 and 54%, respectively. Mild PVY did not significantly (0.01 level ) reduce yield. Monitoring the development of virus symptoms in the field revealed that, in general, more than 50% of the plants exhibited virus symptoms within 33 days following transplanting. Assuming a 10-14 day incubation period prior to symptom development, these plants were actually infected as early as 19-23 days following transplanting. Virus spread continued throughout the season. Studies involving aphid-proof cages revealed that a few seedlings were infected with PVY prior to transplanting. Therefore, the possibility exists that infected seedlings may be transplanted to the field.