Species diversity and complexity of association among species were found essential to the stability and balance of the community system. The community associated with Brassica oleracea L. (Cruciferae) was studied on plants grown in single-species planting and on plants grown in the mixed vegetation of a 15-year fallow field. Approximately 300 species of plants and 3,000 species of heterotrophs existed in the fallow field and these provided species diversity. In 1957, 27 taxa were associated with B. oleracea grown in mixed-species planting and 50 taxa were associated with it in the single-species planting. Three to four times as many parasitic and predaceous taxa were present in the single-species planting. Aphid, flea beetle, and lepidopteran populations reached outbreak levels in the single-species planting, but no such outbreaks occurred in the mixed-species planting. These trends were substantiated in 1958. Thus, it was concluded that species diversity play an important role in preventing population outbreaks. Where possible in cultivation, faunal and floral varieties should be encouraged. The use of specific pesticides and biological control agents to remove pests without sterilization should prove a sound practice.