Selecting the most appropriate test species for sediment and water column assays has been a primary goal for ecotoxicologists. Standard test organisms and established test guidelines exist, but the USEPA-recommended species may not be the most sensitive organisms to anthropogenic inputs. This paper describes preliminary results of toxicity tests with the mayfly, Isonychia bicolor (Ephemeroptera). Results suggested that Isonychia were moderately sensitive to NaCl after 96 h with an average LC(50) value of 3.10 g NaCl per liter. This value decreased after 7 days of exposure, resulting in a mean LC(50) value of 1.73 g NaCl per liter. When exposed to a coal-mine-processed effluent, Isonychia generated LC(50) values that ranged from 13% to 39% effluent. I. bicolor were more sensitive to the coal processing effluent than Ceriodaphnia dubia with conductivity lowest observable effects concentration (LOEC) values for mayfly survivorship that ranged from 1,508 to 4,101 microS/cm, while LOEC values for C. dubia reproduction ranged from 2,132 to 4,240 microS/cm.