Two different effective thresholds for the onset of nonlinearity in the dielectric and piezoelectric coefficients of donor-doped lead zirconate titanate (soft PZT) piezoceramics at electric ac fieldsEc1≈100 V/cmandEc2≈1 V/cmare found. Both are characterized by a scaling behaviorx=x0+A[E−Ec)/Ec]&fgr;above the respective threshold, wherexis the dielectric or piezoelectric coefficient. The values of the effective exponent &fgr; are apparently independent of the particular ceramic system which suggests a universal behavior in soft PZT. We suggest that the nonlinearity is dominated by the dynamics of domain walls in a randomly pinned medium. ©1998 American Institute of Physics.