Gliricidia sepium is a leguminous tree native to Mexico and Central America, and is utilized for fuelwood, animal fodder and green manure in rural communities. It is widely distributed throughout the tropics, but many populations of the tree have arisen from haphazard introductions of unknown quality. Methods for determining the genetic structure and origin of populations are therefore required as a basis for genetic improvement. In order to test the effectiveness of various measures of genetic diversity, provenances of G. sepium from Panama, Guatemala, Mexico and Honduras were studied by electrophoresis of seed storage proteins and the enzymes leucine aminopeptidase (LAP) and acid phosphatase (ACP), and by measurement of plant morphological characteristics. Study of seed storage proteins and morphological characters was very informative, and there was some genetic variation for ACP, but LAP showed no clear variation. In the cases where genetic variation was present, the different methods of study showed a consistent pattern of variation: genetic diversity was greatest in the population from Honduras, and least in that from Panama. This was contrary to expectations on the basis of the locations of origin of the populations and the range in which G. sepium is believed to be native. Electrophoresis of the storage proteins of Gliricidia maculata confirmed the status of this species as a distinct taxon.