Lead filamentary crystals have been subjected to finite deformations. The parameter &dgr; associated with deviations from linearity in Hooke’s law in the form &egr;=s′11&sgr; +&dgr;(s′11&sgr;)2(where &egr; is the strain, &sgr; the stress, ands′11Young’s modulus along the whisker axis) has a value of 4.2±0.6 for four crystals oriented in the ⟨110⟩ direction. The resulting linear combination of third‐order constants is,C111−4.6C112+3.6C123+3.1C144−3.7C166=4.4×1011N/m2.