The effects of dissolved humic acids(HA)on the acute toxicities of the organophosphate pesticide chlorpyrifos and the organochlorine pesticide 4,4′-dichlorodiphenyltrichloroethane (DDT) were assessed by using freshwater crustaceans (Cerio-daphnia dubia) and saltwater crustaceans (Americamysis bahia). The effects of filtered Aldrich HAs (10–100 mg/L) on organism mortality were determined. Humic acids had no effect on mortality of A. bahia for either pesticide at a salinity of 20%, but greatly reduced the mortality of C. dubia for both pesticides in freshwater (0%). In the latter case, the effect was proportional to the HA concentration. The difference in toxicity mitigation as a function of salinity is believed to be due to conformational changes in the HA molecules, which impact pesticide—HA binding, rather than to organismal effects.