Summary An electron microscopic study was made of spermiogenesis and the mature spermatozoon of Phaenocora anomalocoela. During development of the spermatid, the zone of differentiation moves distally from the cytophore. This zone contains basal bodies of two free flagella, an intercentriolar body, extensive flagellar rootlets and a dense central rod lined with microtubules. There is a pointed cap distal to the insertion point of the flagella. The flagella rotate around the shaft to lie adjacent to one another, bend backwards towards the shaft and fuse with it in a distal-proximal direction. The axonemes of unequal length are then fully incorporated in the shaft but are not surrounded by the circle of microtubules which extends along the spermatid shaft. The nucleus, numerous dense bodies and a few mitochondria migrate into the shaft. In mature sperm the distal cap is reduced to a lateral peak, and basal bodies and rootlets are no longer evident. The proximal end of the mature sperm contains only the nucl...