Ram semen, diluted in egg yolk-citrate, and enclosed between pieces of aluminum foil was frozen by immersion in alcohol bath. A freezing rate of 1 °C/min from 5 to –10 °C, 2 °C/min from −10 to −18 °C, 4 °C/min from −18 to −35 °C and 5 °C/min from −35 to −70 °C, was employed. This treatment was chosen because of the recovery of a relatively high percentage of frozen-thawed sperm in an earlier investigation. The ice was removed by freeze-substitution and the specimen infiltrated with Epon for electron microscopy. The acrosome appeared empty in frozen-thawed spermatozoa. The nucleus showed no evidence of intracellular freezing. The mid-piece region of approximately 50% spermatozoa contained ice cavities. These cavities distorted the inner fibers and the cristae in the mitochondria. These ultrastructural alterations may be the visible manifestations of alterations such as the leakage of certain isozymes of acid phosphatase. These changes appear to affect the fecundity of spermatozoa.