Forty-one species of Rotifera are recorded from the River Sokoto, North Nigeria, and details of their seasonal abundance are given. There is a marked change in the composition of the rotifer fauna of the river when it floods in the single wet season. The cyclomorphosis of Keratella tropica begins with the appearance of the species in December. At this stage most individuals lack a left posterior spine. The left posterior spine then grows longer in successive samples until February or March and then decreases to very short lengths in June. The right posterior spine does not vary much throughout the cycle and is always positively correlated with the length of the lorica. The length of the left spine shows varying degrees of correlation with the length of the lorica according to the rate of change of length of the left spine. Brachionus caudatus shows synchronous changes of both its posterior spines, the lengths of which are positively correlated with the length of the lorica. The relation of these findings to the general theories of cyclomorphosis is discussed. The flotation theory of Wesenberg Lund is rejected from application to the species studied. The necessity of considering cyclomorphic phenomena in relation to body size is emphasized.