The optimum temperature for germination of the conidia of Colletotrichum coffeanum under conditions of low nutrition was around 22°C., but the optimum broadened towards the higher temperatures with improved nutrition. The optimum temperature for lesion formation on green coffee berries was rather lower than that for germination of conidia. The range over which it is possible for germination and infection to occur lies between 17 and 28° C. Although germination and infection can take place under humidity conditions approaching saturation, the requisite combination of near-saturation with suitable temperatures rarely occurs in the field, and then, usually, for too short a time for infection to be completed. It is concluded that the presence of liquid water, resulting from either rain or heavy mist, is necessary for infection under field conditions. This study has enabled an explanation to be advanced for the apparent restriction of the disease to certain altitudes, and also for the observed relations between disease incidence and such factors as topography and cultural practices, e.g. the use of overhead shade. Consequently, standard meteorological data can be used to predict the probable incidence of the disease in any given district.