The sperm flagella of the eel, Anguilla anguilla, are capable of vigorous motion in spite of having an axoneme with reduced structure that lacks the outer dynein arms, radial spokes and spoke heads, the two central tubules and the central tubule projections that are all part of the standard "9+2" axoneme. These sperm progress forward rapidly as a result of the propagation of helicoidal waves distally along the flagellum. Their flagellar beat frequencies are high, 93 Hz at 21 degrees C, and they roll at a frequency of about 19 Hz. Eel sperm could be demembranated with Nonidet P-40 and reactivated with MgATP2- in 0.22 M K acetate at pH 8.1. The reactivated motility closely resembles that of the live sperm, with a beat frequency of 69 Hz, but the demembranated flagella are unusually fragile, and commonly disintegrate by a combination of splitting, coiling, and sliding within a few minutes. Little reactivation is obtained if acetate is replaced by Cl- in the reactivating medium. The Michaelis constant for beat frequency (0.2 mM) is similar to that obtained for several "9+2" flagella. These sperm, however, appear to lack the mechanism by which Ca2+ regulates waveform. Our results indicate that eel sperm flagella, which at rest are straight, are induced to bend helicoidally by ATP, as the result of sliding between tubules that is blocked at both the base and tip of the organelle. The flagellar waveform consists of a series of planar bends separated by short regions of right-handed twist, which give it an overall left-handed helicoidal form.