In this work the spermatozoon of Chloëon dipteron (Ephemeroptera) is examined. The three most important features are that (a) the acrosome is very brief; (b) the mitochondrial derivative is represented by a long mitochondrion with transverse cristae and by a crystalline mass enveloped in a separated membrane; (c) the axial filament of the tail lacks the two principal central units and the central sheath, and is therefore, of the 9 + 9 +0 pattern. This type of flagellum has not previously been described. © 1969 Academic Press, Inc.