Among 34 wind-dispersed tree species on Barro Colorado Island, Panama, the wet mass of diaspores ranges over six orders of magnitude, The seed mass as a percentage of diaspore mass (S/D) varies greatly among species from 14 to 94% with a mean value of 61% The mean percent moisture of diaspores is 10% no consistent differences occur between seed and non-seed components of diaspores in percent moisture. Wing-loading (weight/area) and hence dispersal capacity varies over one order of magnitude among these species and is correlated more highly with S/D than with percent moisture of diaspores. Compared to fruit diaspores, seed diaspores have less mass and a greater S/D, and are slightly more dehydrated. As a result, seed diaspores have lower wing-loading and greater dispersal capacity than fruit diaspores. The morphological/aerodynamic features of a diaspore also have a significant effect on the mass variables. Overall, the type of diaspore (fruit or seed) is of major importance in explaining mass differences of diaspores among these wind-dispersed species; morphological features and dehydration ability contribute secondarily.