Near-isogenic lines of mungbean (Vigna radiata) were produced from backcrossing two Thai recommended cultivars, KPS 1 and CN 36, with the multiple leaflet line, V 5926, until BC9. The isogenic lines were evaluated against the two recipients at Kasetsart University, Thailand, in the rainy season of 1996 and the dry season of 1997. The effect of the gene controlling the multiple leaflet on physiological and agronomical characters was investigated. Seed yield, plant height, and number of pods per plant were greater in the trifoliate leaflet cultivars than in the multiple leaflet near-isogenic lines, whereas the numbers of seeds per pod and 1000-seed weights were not significantly different. Light saturation and photosynthetic rate did not differ, while light interception and dry matter accumulation were greater in the trifoliate leaflet cultivars than in the multifoliate lines. The leaf area index (LAI) of the trifoliate parents was greater at the vegetative stage but lower at the flowering and young pod stages, due to the greater number of leaflets per plant of the multifoliate lines. After the beginning of the podding stage, the LAI of the multifoliate lines was less than that of the parents, as the number of leaflets per plant of these lines hardly changed while that of the trifoliate parents increased.