The two goldenrod leaf beetles, Trirhabda virgata LeConte and T. borealis Blake (Coleoptera: Chrysomelidae), flew only during a brief, postteneral period, when each population was at peak density. Cessation of flight coincided with the period of rapid maturation of oöcytes. The sex ratio of flying individuals was strongly skewed toward females, although males and females were equally abundant in each population at large. Most beetles were captured just above the goldenrod vegetation (at 1 m above ground); the number of captures decreased with increasing height above ground. Females tended to fly higher than males. The role of dispersal by flight in Trirhabda habitat colonization and population dynamics is discussed.