A study on the effects of succinic acid-2, 2-dimethylhydrazide (SADH) and populaton density on growth and yield of groundnuts was conducted at the experimental field of the Faculty of Agriculture, Nakhon Si Thammarat During June to September, 1988. Two groundnut varieties. namely Taninan 9 and SK. 38, were used in two separated experiments. In each experiment, SADH was applied at the rates of 0, 192 and 288 grams/rai, at 45 and 55 days after emergence and 20, 40 and 60 plants/m(2). The experiment was conducted in a 3 x 2 x 3 factorial combinations in a randomized complete block design with four replications. The result showed that application of SADH reduced plant height, mainstem length as well as primary branch length in both groundnut varieties. This was due to shorter internodes and the reduction in number of nodes. Shoot dry weight and leaf area per plant also decreased but showed no effect on branching, number of seeds per pod and seed yield. The application of SADH to Tainan 9 showed no effect on shelling percentage, 100 seeds weight, pod yield and seed yield. However, SADH at the rate of 192 grams/rai increased number of mature pods and harvest index while no significant effect was found at the rate of 288 grams/rai. The increase in number of mature pods per plant and harvest index dur to the application of SADH at the rate of 288/grams/rai was found to be contributable to pod yield of SK. 38. On the other hand, the shelling percentage and 100 seeds weight of this variety tended to decrease : therefore, no significant difference on seed yield was observed. The application of SADH at 55 days after emergence incrased pod and seed yields for Tainan 9, but this was opposite to SK. 38. The high population densities of 40 and 60 plants/m(2) promoted plant height, internode length of mainstem and of primary brancheds. However, these densities for both varieties decreased number of nodes on mainstem and primary branches, number of brahches per plant, shoot dry weight, leaf area per plant and number of mature pods per plant as compared with the conventinal plant population of 20 plants/m(2). The conventional plant population of 20 plants/m(2) gave the highest pod yield. However, various population densities did not show significant difference on seed yield. In the analyses of interactions among treatments, it was found that the application of SADH at the highest rate (288 grams/rai) and at 45 days after emergence significantly decreased the internode lenght of the mainstem and the length of primary branches in both varieties. Moreover, this treatment significantly decreased the internode length of primary branches and the mainstem length of Tainan 9. The highest application rate of SADH in combination with the lowest population density (20 plants/m(2) decreased the internode length of primary branches and hence the branches length of SK. 38. This showed that SADH exhibited some effects on SK. 38. The applications of SADH with both levels on the lowest plant density and the highest level (288 grams/rai) on the highest plant density (60 plants/m(2) significantly decreased shoot dry weight in SK. 38.