Four hundred and seventy-two Australian peregrine falcon eggs were measured. A mean decrease in shell thickness of 10.4%, and a maximum of 38%, were found in eggs collected after 1947. Thinning commenced in 1947-49 and corresponded temporally with the introduction of DDT use in Australia and to similar thinning in peregrine eggs internationally. The proportion of eggs with a decrease in shell thickness of 20% or more has increased since 1947-49. The degree of thinning is associated geographically with land use type and DDT use in crop production. Various other factors which may contribute to variation in eggshell thickness are examined and the relationship of thinning to decreased productivity in certain areas is discussed. Victoria appears to be the State with the highest mean thinning.