AbstractStudies undertaken to determine the causes of continuing reproductive failures of populations of peregrine falcons (Falco peregrinus anatum) in Texas and California demonstrate exceptionally high levels of DDE (1,1‐dichloro‐2,2‐bis(p‐chlorophenyl)ethylene) in prey species in western Texas and unchanged high levels of DDE since 1969 in the peregrine population of the central coast of California. Although DDE is a derivative of DDT (p,p'‐DDT; 1,1,1‐trichloro‐2,2‐bis(p‐chlorophenyl)ethane), levels of DDT were very low in in these samples. These observations follow a recent pattern of DDE contamination in the South and Southwest that is characterized by locally high environmental levels of DDE and low levels of DDT. The virtual absence of DDT indicates that the possible illegal uses of DDT, or the transport of DDT compounds from Latin America by migrating birds, cannot account for all of the DDE in these samples. Another source of environmental DDE is th