Using 2 closely related species, holbrooki of the Atlantic drainage and affinis (or patreulis) of the Mississippi Valley, 103,150 individuals of 21 mm. (using "eye measure") and over in length were sexed, principally on the basis of the anal fin if the anal fin had become changed into an intromittent organ (gonopod) the fish was classed as [male]; if not, [female] . Later 1945 fish 20.5-30 mm. long with unmodified anal fins and showing no dark abdominal spots, as in gravid [female], were sexed by examination of internal sex organs. Finally, 2593 immature fish (from birth (about 8 mm.) to 20 mm. long) were sexed by examination of internal sex organs. Among the adults (21 mm. and upward in length), a seasonal variation was found. The ratio in midsummer (Aug.) was 1 [male] to 11.31 [female]; during early summer (June), 1 [male] to 2.54 [female]. The results confirm those of earlier investigators (Barney and Anson); the explanation of these authors, however, is rejected. From the examination of specimens 20.5-30 mm. long, which could not be sexed from external characters, it was found that a portion of [male] had not developed the intromittent organ when 21 mm. long. In fact, a few [male] fish 28 mm. long had it undeveloped, but none above that size. Results show that the number of fish classed as [male] from examination of external sex characters, is somewhat below the actual, but the error in sexing is too small to account for the large differences in sex ratio obtained. In the young (20 mm. and under in length), the sex ratio was nearly 1 (1298 [male] and 1295 [female]). Evidence is adduced that the [male] are the weaker sex, that they possess less resistence to unfavorable environment, and that they probably also are more extensively preyed upon. Because of the abundance of Gambusia in favorable environments and their very rapid multiplication under such conditions, and because recopulation between the broods of a [female] during [male] season is not necessary, apparently enough d" for reproductive purposes are always present.