1. Dimorphic spermatozoa are produced and shed by the rotifers, Brachionus mulleri, Asplanchna amphora, Polyarthra platyptera, Hydatina senta, Diglena catellina, Euchianis dilatata, Metopidia lepadella, Brachionus urceolaris and Brachionus bakeri. 2. One class of the spermatozoa is composed of large, vermiform, and motile spermatozoa and the other class is composed of small, spindle-shaped and immotile spermatozoa. 3. The number of motile spermatozoa produced is exactly twice the number of immotile spermatozoa produced. 4. The motile spermatozoa probably develop from the cells formed by the second division of the normal spermatocytes but the immotile spermatozoa probably develop directly from the rudimentary cells formed by the first division of the spermatocytes. 5. The motile spermatozoa are probably functional but the immotile spermatozoa are probably rudimentary and functionless. 6. The functional spermatozoa are all identical in their power of determining the sex of the individual that develops from a fertilized egg. After a functional spermatozoön has fertilized a parthenogenetic male egg the egg eventually develops into a female individual instead of developing into a male individual as it otherwise would have done if it had not been fertilized.