The demography of breeding populations of the longnose garLepisosteus osseushas heretofore remained unstudied. During the summer of 1992, longnose gars were captured en route to spawning sites in Weaubleau Creek, a tributary of Missouri's Harry S. Truman Reservoir. To characterize the spawners, individual age, growth, and morphometric data were obtained. The spawning population consisted of smaller, younger males and somewhat fewer older, larger females. As expected, Weaubleau Creek spawners exhibited heightened condition values relative to other longnose gar populations assessed outside the breeding season. Females displayed greater backcalculated total lengths at age and higher growth rates than males, with these differences being manifested early in life. Examination of individual yearly growth increments suggested that most longnose gars in Weaubleau Creek were annual spawners. Females made substantial investments of time and biomass in reproduction, as their gonad weight–body weight relationships, fecundities, and spawning-associated weight and girth losses reflected. Males also invested heavily in reproduction, but may have recouped some biomass through postspawning instream foraging. Fish from nearby Truman Dam differed in their sizes, condition, growth, relative gonad sizes, and relative branchiostegal ray sizes, possibly reflecting the impact of earlier maturation associated with abundant forage. A novel approach for assessing breeding participation in fishes is also developed.