Maintaining species diversity in the nearshore, intertidal, and freshwater systems inhabited by the many kinds of killifish has become increasingly important as the twentieth century draws to a close. Because of their incredible ability to withstand a wide range of temperatures, salinities, and pollutants, especially organochloride pesticides and fertilizers, killifish seem to be among the most persistent species. In 1990, five species of killifish were the only fish remaining in Mullet Pond, North Carolina, which had 27 fish species present in 1903 and 1914. In the 1960s, the entire remaining population of the Owen's Valley pupfish (Cyprinodon radio-sus) was transferred in a few buckets by Fish and Wildlife agents, as their native pool drained away. Perhaps because of their widespread distribution and ability to adapt to whatever conditions exist, the killifish are among the most important fish species for monitoring the future of our environment.