Effects of population density of mungbeans on differentt characters were studied in the early and late rainy season in 1987 at Prince of Songkla University using two lines and a cultivar of mungbean, namely PSU 424-61, VC 1973A and U-thong 1. Each study was conducted in a split-plot design with three replications. Mungbean lines and cultivar were used as main-plots and populations of 5, 10, 20, 30, 40, 60, 80 and 100 plants/mm as sub-plots. In the early rainy season, mungbean line PSU 424-61 and the population of 60 plants/mm tended to give the highest yield of 180.98 and 190.33 kg/rai, respectively. For the late rainy season, however, U-thong 1 and the population of 80 plants/mm gave the highest respective yield of 212.79 abd 247.08 kg/rai. For conventional planting rate (20 plants/mm), PSU 424-61 tended to give the highest yield of 219.81 kg/rai in the early rainy season, whereas, U-thong 1 tended to give the highest yield of 167.10 kg/rai at the late rainy season. Yield components including pods/plant, pod lenght, seeds/pod and seed size were not different among lines and cultivar under study. Increases in population densities resulted in the decrease of pods/plant, pod length and seeds/plant of all entries at both planting dates. However, increased population densities did not significantly affect seed size of mungbeans. No significant difference among cultivar and/or lines was found for days to first flower, leaf area/plant, dry matter/plant, plant height but disease resistance. However, all the characters were significantly affected by population densities. That is, increases in population densities resulted in the decreases of leaf area/plant and dry matter/plant, but increases of days to first flower, plant height and disease and lodging ratings. Correlation analyses showed the significant association between seed yield and plant population for U-thong 1 in the early rainy season and for VC 1973A and U-thong 1 in the late rainy season. Significant correlations were also found between pods/plant and seeds/plant, pod lenght and seeds/plant. Correlations between ps/plant and plant population, seeds/plant and plant population were negative.