THE upward forces produced by the heterocercal tails of two species of Selachii, moving transversely through water at various speeds, have recently been measured1. It was found that the lift was approximately proportional to the 1.4th power of the transverse speed. The equilibrium of the fish, swimming horizontally, was discussed, and it was estimated that in each case the lift required for equilibrium would be obtained at swimming speeds around 0.5 body lengths/sec. At higher speeds, more lift would be obtained if the hypocaudal lobe acted passively, but it was thought probable that the lift could be adjusted to the required value by means of the radial muscles of the hypocaudal lobe.