Hemagglutinating activity (HA) of the serum of the 5^th-instar desert locust Schistocerca gregaria was detected against a range of vertebrate red blood cells (RBCs). Serum strongly agglutinates RBCs of rabbit. In contrast, no agglutination was observed for RBCs from guinea pig or horse and a very low level was observed for human, sheep, and rat RBCs. HA is Ca² dependent, heat-labile, and was strongly inhibited by α-linked-D-galactosides and rhamnose. Developmentally, a relatively low level of HA (≤32) is present in the extracted fluids from centrifuged homogenate of 1 to 12 day-old eggs. Also, a limited level of HA was observed in the 2^nd and 3^rd instars. However, prominent and cyclical patterns were observed in the 4^th and 5^th instars. In each of these cycles, HA starts low (64), maximizes (128, 512, in the two instars respectively) at about midstadium, then declines again prior to ecdysis to the next stage. On the other hand, adult stage HA starts at a low value (512), then maximizes and is sustained at a fairly constant value (1024), without any difference in the sexes throughout the period of measurements (up to day 73 of the life cycle). HA is independent of sex and season but conspicuously and reproducibly varies with stadium, stage and age.