Histological studies of bean rust colonization revealed that mesophyll colonization accelerates abruptly on the sixth day following inoculation corresponding to (a) the time of development of the first visible symptoms and (b) the often reported manifold increase in respiration of diseased leaves. There is an accompanying hypertrophy of mesophyll cells. These observations are considered as further evidence that much of the respiratory increase may be ascribed to the fungus but they do not discount host contributions. With scanning electron micrography and standard histological techniques, new observations made on the early stages of pustule development cast doubt on the (assumed) role of tactile pressure in uredial opening.