Sectioned antheridia of Sphagnum have been investigated with the electron microscope at magnifications up to 50, 000 diameters and anatomical features of the main protoplasmic organs in two types of cells are described and illustrated. In the parenchymatous cells of the antheridial wall the main components encountered are: nucleus, starch-laden plastids, mitochondria, fat bodies, tubules, putative golgi areas, vesicles, membranes, and granules. In the coiled spermatozoid the main observations concern the coiled nucleus, the vestigial plastid, putative mitochondria at the two extremities of the cell, the distribution of vesicles and fibres in the cytoplasm, and the structure and mode of attachment of the two flagella. In a comparative discussion it is suggested that the parenchymatous cells show resemblance to cells of the higher plants, notably in details of their plastids and mitochondria. The extreme specialization of the spermatozoid impedes close comparisons with other plants but a homology is tentatively suggested between a peculiar organ referred to here as the ‘fibrous band’ to which the nucleus and both flagella are attached and certain intercalary fibres originating in a comparable position between the bases of the flagella in green and brown algae.