The ultrastructure of the mature sieve elements of the Saxifraga stolon is described. Theseare narrow (3–5 μm) and fairly long (100-300 μm). The sieve plate pores were invariably found to be closely-occluded with P-protein. It is argued that the double-cutting technique used for excision invalidates the contention that this must be interpreted as an artifact. The P-protein filaments appear to consist of a double helix. Stereo micrographs at normal voltages, and at 1 MV in the AEI-EM₇ microscope are presented.