The mating system of Sesbania sesban was studied using one natural population
and two cultivated populations. A large number of progeny (600) was assayed from each
population for one polymorphic locus of Malate dehydrogenase (Mdh). The level of out-
crossing in the studied populations ranged from 40-100 % . The genotype frequencies were in
agreement with those of the Hardy-Weinberg expected genotype proportions as in randomly
mating populations at P < 0.05. The floral structure and the pollen shedding behaviour of
the flowers of S. sesban could favour cross-pollination and/or selfpollination depending on
the circumstances. Self-pollination usually occurs late in the flowering period when
pollinators fail to visit the flowers. Such a "delayed selfing" facilitates out-crossing in
the presence of appropriate pollinators under natural conditions, but confers a selective
advantage where pollinators are limiting. The results indicated that S. sesban could be
self-fertilized in the absence of pollinators or could out-cross with other heterozygous
individuals in the neighboring population when suitable pollinators are available. S.
sesban exhibits "facultative xenogamy" or "delayed selfing" and is a mixed mating species.