No trace of morphological heterospory has been found in the genus Equisetum. Reports that sex may be altered by environmental conditions indicates that it is not determined by genotypic or physiological properties of the spores. Spore size is extremely dependent on viability, maturity and mounting medium, but within each species all the viable spores are spherical, bright green and vary little in size. Slight subgeneric differences exist in spore size but there is great overlap between all species rendering the spores of little use taxonomically. Colourless abortive spores are found in the two hybrids E.×moorei and E.×trachyodon and also in E. telmateia and E. palustre var. polystachyum. Of common occurrence in E. hyemale, E. ramosissimum and E. laevigatum are small, discoloured, misshapen spores which will not germinate. Considerations of the phenology of these species in relation to experiments on storage of ripe cones indicate that these spores were at one time mature and viable but had subsequently died. In E. variegatum and E. scirpoides large variations were observed between cones, in spore size and germination. This was due to the ripe cones containing variable proportions of mature and immature spores.