An experiment was performed to investigate the spread of infection in maize streak virus epidemics. Four plots, containing 40 rows by 60 stands of maize were planted at two different locations in Uganda, and in four different seasons and the incidence of maize streak virus disease was mapped in the plots every 7 or 14 days from sowing until tasselling. The logistic curve was a good description of disease progress in each plot which, may indicate the importance of secondary spread. Spatial aggregation of diseased plants in the plots was analysed using both distance class analysis and the beta-binomial distribution. Diseased stands were found to be highly clumped. New foci were generated throughout the season. The degree of aggregation increased considerably at the end of the season as these foci coalesced. These findings may be related to the mating behaviour of the vector (Cicadulina spp.) and heterogeneity in crop height where males find single females stationed on plants of a preferred height. Females move out of the crop after mating but a succession of females attracted to plants in the same part of the field could cause apparent expansion of foci. As plant heights change then new disease foci will be generated.