A series of four separate experiments was conducted with the following major aimes. 1. To evaluate the occurrence of competition between existing Zoysia japonica and oversown pasture species under field conditions. 2. To examine the toxic effects of leachate produced from underground parts of Zoysia japonica in water cultures. 3. To determine the effects of root and shoot competition of Zoysia japonica on the establishment of orchardgrass seedlings in pot cultures. 4. To evaluate the roles of both soil moisture and nitrogen as root competitive factors between existing Zoysia japonica and oversown orchardgrass seedlings in pot cultures. These experiments were designated and described as Experiment 1 - Field Test, Experiment 2 - Water Culture Test, Experiment 3 - Pot Test and Experiment 4 - Pot Test. Three species, orchardgrass, ky 31 fescue and ladino clover were used in Experiment I, the above two grasses and Italian ryegrass were also used in Experiment 2 and only orchardgrass was used in Experiment 3 and 4 as test plants. However, as an aggressor plant, Zoysia japonica was used in all experiments. With one exception, three experiments were carried out in a semi-controlled vinyl house environment and all work was done at the Livestock Farm, College of Agriculture, Seoul National University, Suwon, Korea. The summarized results of each experiment are as follows; Experiment 1. A study of factors influencing grasses and clover establishment and survival on native pastures dominated by Zoysia japonica was made on infertile native grassland and also an evaluation was made of the occurrence of competition between existing Zoysia japonica and oversown pasture species. Nitrogen and lime (Ca(OH)₂) depressed significantly establishment of clover. Lime depressed clover establishment was probably initially due to a strong alkalization of surface soil. Lime, nitrogen and phosphorus had no significant effect on initial establishment of grasses in bath presence and absence of Zoysia japonica, but lime aided clover survival in both presence and absence of herbage cover and phosphorus aided survival of grasses in absence of cover. Dense herbage cover had little effect on clover establishment and survival, indicating no competition between clover and existing Zoysia japonica, but had a depressing effect on grass establishment and survival, indicating competition between sown grasses and Zoysia japonica. It is considered that failure of grasses to become established amidst dense Zoysia japonica may be attributed to competition from existing Zoysia japonica. Experiment 2. To determine the mechanism of root competition by Zoysia japonica experiments were carried out on the production of toxins from its underground parts. Zoysia japonica was used in the study mainly because of its reputation as a strong competition in a previous experiment. In these experiments to test the production of toxins from Zoysia japonica, seedlings of orchardgrass, ky 31 fescue and Italian ryegrass were treated with leachates of roots and rhizomes, and seeds and seedlings of the above grasses were germinated and grown in close contact with living Zoysia japonica roots. These experiments showed that there were no evidence of any appreciable growth promotion or inhibition by Zoysia japonica. Therefore, it is evident that the competition between Zoysia japonica and oversown grasses is not due to the production of toxic substances from Zoysia japonica. Experiment 3. The effects of root and shoot competition of Zoysia japonica on the ability of orchardgrass seedlings to become established in native pastures dominated by Zoysia japonica were studied. These studies were carried out during 1971 in vinyl house pot cultures using a split plat design, with three moisture levels imposed on five competition treatments including (1) full competition (2) root competition (3) shoot competition (4) no competition and (5) mono culture. Soil moisture treatments were; (