The Leguminous Leucaena including 15 species, of which several have become pantropical as fodder, fuelwood, shade and ornamental trees. 160 collections of 12 Leucaena species, including 3 tetraploids and 9 diploids, were analyzed for six isozyme systems. Extensive inter and intraspecific variability was detected using cotyledon tissues. The uniformity of isozymic expression in 80 collections of the “common leucaena” from all over the world supported the hypothesis that it is a single self-pollinated variety. It was first distributed to the Philippines from Mexico in late 1500s, then spread worldwide. ACO polymorphism appeared to be controlled by three loci ACO1, ACO2, ACO3 and IDH by two loci IDH1, IDH2 in L. lanceolata. Peroxidase polymorphism was early shown to involve 4 loci, PX1-PX4. Polymorphism in these loci was applied to clarify phylogeny of the Leucaena ssp. and hybrids (>60 made in Hawaii), that are growing in the Waimanalo Sta. of UH.