The ratio of females to males in pseudergates, soldiers, nymphs, and alates of Reticulitermes flavipes (Kollar) was 1.03:1, 1.27:1, 1.14:1, and 1.12:1, respectively. Only in soldiers was the percentage of females significantly greater than that of males. The longest oocyte in an ovary increased significantly in size when pseudergates developed into nymphs, when nymphs developed into alates, and when alates were aged for a period of 10 d. Mature oocytes were observed only in the ovaries of 10-d-old de-alates and tertiary reproductives. Spermathecae also increased significantly in size as pseudergates developed into soldiers or nymphs and nymphs developed into alates. Sperm was not observed in the spermathecae of swarmers but was present in 10-d-old de-alates. Significant increases in length of the testicular lobes were observed as pseudergates developed into nymphs. The testicular lobes of tertiary reproductives were about three times the size of those of nymphs, swarmers, or 10-d-old de-alates. Sperm was observed in the ejaculatory ducts of soldiers, nymphs, alates, and tertiary reproductives.