AbstractDDE (1,1‐dichloro‐2,2‐bis(p‐chlorophenyl)ethylene), the DDT derivative that causes thinning of the eggshells of sensitive bird species, is metabolically formed in mallard ducks fed Kelthane®, a widely used acaricide that consists of technical dicofol, a mixture of compounds synthetically derived from technical DDT. Technical dicofol mixtures contain very low levels ofp,p'‐DDT, which has been assumed to be the only precursor to environmental DDE. Applications of Kelthane and of other products containing technical dicofol would therefore increase the environmental input of DDE, accounting at least in part for recent observations of high levels of DDE in environmental samples from Texas, New Mexico, Arizona and California that contained only trace amounts of DDT. Among the components of Kelthane, chloro‐DDT (1,1,1,2‐tetrachloro‐2,2‐bis(p‐chlorophenyl)ethane) is converted to DDE under certain laboratory conditions. Levels of chloro‐DDT in the Kelthane mixture employed could account for all of the DDE metabolically formed in the ducks. The data leave unresolved the question of whether DDE may be derived fromp,p'‐dicofol (1,1‐bis(p‐chlorophenyl)‐2,2,2‐trichloroethanol), the principal ingredient of technical dicofol, under either experime