Vegetative cells of a Cyclotella clone (03A) form male gametes and auxospores in response to salinity changes in the medium. Two different valve patterns which characterize described species of diatoms, Cyclotella meneghiniana and C. cryptica, may occur during clonal development of the auxospores of clone 03A. Evidence is presented which indicates that the valve morphology is environmentally controlled. This polymorphic clone is distinguishable from either species on the basis of the morphology of its male gamete. It has a male gamete which is anteriorly uniflagellate. The male gametes of both C. meneghiniana and C. cryptica are posteriorly uniflagellate.