The capillaries of the frog mesentery were investigated by transmission electronmicroscopy. The mean diameter of these vessels was 20 micrometers and a single layer of five to six endothelial cells (on average) contributed to each circumference. Their ultrastructure was very similar to that of mammalian capillaries of the continuous type [Bennett, Luft and Hampton, 1959]. The endotherlial cells, which contained many vesicles, were joined by specialized junctions and surrounded by a continuous basement membrane. Eleven capillaries were examined after the filtration coefficient of their walls had been investigated by the microperfusion micro-occlusion technique of Michel, Mason, Curry and Tooke [1974]. No abnormal appearances were observed even after the vessels had been perfused with protein free solutions which have been shown to increase filtration coefficient five fold. No morphological differences between capillaries were seen to accompany large differences in the filtration coefficients.