The influence of ultraviolet (UV) radiation in simulated natural sunlight on the viability of ascospores of Mycosphaerella fijiensis, the cause of Black Sigatoka disease in banana and plantain, has been investigated as part of a study to assess the windborne spread of this pathogen from mainland Central and South America into the Caribbean. Spores were killed following continuous exposure to UV radiation for periods of 6 h or over. This relatively short exposure time suggests that the distances over which viable spores can be transported will be determined not only by the speed of the wind but also the amount of cloud cover and the time off day that spore release occurs. On this basis, wind dispersal of viable spores over distances greater than a few hundred kilometres is unlikely. These conclusions are reinforced by an examination of historical reports of the arrival of the disease in previously uninfected areas of the Americas and Africa.