AbstractSpermatozoa of abaloneHaliotis discuswere examined before and during the acrosome reaction with special regard to one of the newly formed structures: a cylindrical structure surrounding a part of the elongated acrosomal process near the opening of the acrosomal vesicle. The structure, about 0.2 μm in diameter and about 1 μm in length, was revealed to be composed of a tightly coiled, fine tubular structure about 20 nm in diameter. In the course of the acrosome reaction, a triple‐spiral structure appeared in the anterior part of the acrosomal vesicle. Since this spiral structure was also composed of a tightly coiled 20 nm tubule(s), it was concluded that this structure was transformed into the single‐walled cylindrical structure by simple stretching in the direction of its longitudinal axis. In the clumps of spermatozoa that underwent acrosome reaction in suspension, the cylindrical structures were frequently found in contact with each other and/or other structures, indicating that they are very s