It has been found that spermatozoa of bull, rabbit and man swell in hypotonic media and in media of penetrating non-electrolytes. 1. 1. When the volume of the spermatozoon increases, due to osmotic uptake of water, the cell membrane takes on a more and more spherical shape, apparently without a significant enlargement of its area. 2. 2. The flexible “motor apparatus” of the tail is hereby forced by the cell membrane to bend and coil. 3. 3. When the cell has attained a critical volume the cell membrane ruptures and the coiled “motor apparatus” is abruptly straightened out. 4. 4. Calculations indicate that bull spermatozoa with maximally swollen tails might have increased their volumes 3.5–4 times. 5. 5. The plane of bending and coiling of the “motor apparatus” of the bull spermatozoon is determined by the plane sectioning fibre 1 and passing between the central fibres and fibres 5 and 6. 6. 6. Swollen spermatozoa loose water, straighten out (“uncoil”) and regain normal flagellation when the salt concentration of the medium is raised to isotonicity. 7. 7. Swelling manifests itself in a lowering of the specific gravity of epididymal bull spermatozoa. 8. 8. The close association of the cell membrane with Jensen's ring, which normally may prevent the cytoplasmic droplet from a migration down the principal piece, does not influence the alteration in shape of the cell when its volume is increased. 9. 9. Coiled spermatozoa in ejaculates apparently are the result of osmotic swelling.