We investigated relationship between behavior of chemical substances and toxicity in some basins dominated by residential area. We used GC/MS simultaneous analysis database for chemical analysis, and Medaka (Oryzias latipes) early fry bioassay using concentrated sample water for toxicity analysis. Conclusions were: 1. There were some rivers which contained chemical substances at the similar level as 50% fish Lethal Concentration (LC₅₀) after 100 fold concentration. 2. It was difficult to explain toxicity on fish using known LC₅₀ and chemical concentration obtained by GC/MS analysis. 3. Although most chemical concentration were relatively low in residential areas, toxicity became higher in basin of low sewered population. 4. When ratio of light industrial and/or commercial district was larger, chemical concentration tended to become higher. At the same time, toxicity also tended to become higher. 5. Composition of chemicals varied significantly within a short flow path of the rivers.