Studies on the flagellar movement of carp spermatozoa induced by dilution in distilled water allowed us to describe a sequence of early, rapid morphological and kinetic changes which begin at the very tip of the flagellum. They cause the progressive folding of the axoneme which ends stuck to the head within 90-120 seconds after the initiation of motility. However, the axonemal machinery remains functional as the folding can be reversed after transfer back into a high osmolality medium and partially folded flagella were able to propagate efficient waves along the non-folded proximal portion of the axoneme. The data also revealed that the membrane area of the terminal piece exhibits strong sensitivity to hypotonicity. These results suggest that in the normal freshwater medium, the brief swimming period allowing fertilization of oocytes is limited by the osmotic stress induced coiling of the carp sperm tail and not by ATP stores.