The toxicity, absorption, distribution, metabolism, and effects on reproduction of DDT was studied using the killifish (Fundulus heteroclitus), a species of economic importance because of its widespread abundance and its presence toward the lower end of the food chain. 14C-DDT was administered by exposure from the ambient water. There was a rapid removal of the radioactive pesticide from the water accompanied by uptake of radioactivity primarily by carcass (primarily muscle tissue) and eggs of the fish. Most (greater than 92%) of the radioactivity in the carcass was shown by TLC methods to be the parent pesticide. One day after a single 24-hr exposure to 14C-DDT, approximately 70% of the administered radioactivity was found in the carcass and the levels of the tissue decayed with a t 1/2 of three days. One day after a single 24-hr exposure to 0.1 ppm of 14C-DDT, the organs that contained the highest concentration of the pesticide (ca. 5 ppm) were intestine and liver. When the pesticide was administered by two 24-hr exposures from water, the intestine, liver and ovaries contained the major concentration of radioactivity (7 to 14 ppm). Untreated Fundulus contained less than 0.2 ppm of total DDT-like compounds. A variety of doses and schedules were tested in an effort to maximize the absorption of DDT, while minimizing the mortality to the fish. An intermittent schedule of 24 hr in 0.1 ppm DDT followed by 24 hr in DDT-free sea water, repeated two times, was found to be optimal. At the levels examined, DDT delayed the rate of normal development of fertilized eggs from Fundulus, but did not appear to cause any observable alterations in the hatched fry. Fertilization of Fundulus eggs was significantly diminished when insemination was carried out in DDT-containing sea water.