Killifish (Fundulus heteroclitus) eggs from a polluted creek (Piles Creek (PC)) and a relatively pristine estuary in Long Island (LI) were exposed for 20 min to various concentrations of methylmercuric chloride (MeHg) prior to combination with untreated sperm. PC killifish eggs showed a higher LC50 value (1.7 mg/liter) than LI eggs (0.7 mg/liter). PC eggs that were fertilized by nontreated sperm after exposure to 1.0 or 2.5 mg/liter meHg and then placed in clean sea water (15 parts per thousand) for 1 week showed a 5 and 7% malformations of the embryos, respectively. However, exposure of LI eggs to 1.0 mg/liter prior to fertilization caused 32% malformations of the embryos, and at 2.5 mg/liter almost all the embryos died. The data indicate that LI killifish eggs are less tolerant to meHg than PC eggs. This is in keeping with previous data on embryonic tolerance to meHg in these two populations. However, 96-hr LC50 values of juvenile fish (25-45 mm standard length) did not differ between these two populations.