The second hyperpolarizability ⟨&ggr;xxxx⟩ of a new, soluble, conjugated polythiophene derivative, containing aromatic and quinoid moieties, is measured to be 1.2×10−31esu. The measurement was made at 532 nm in a dichloromethane solution by picosecond‐resolved degenerate four wave mixing. The corresponding macroscopic third‐order susceptibility &khgr;(3 )is estimated to be 4.6×10−9esu, which is larger than that of the parent poly(2,5‐thiophene) and many currently known nonlinear optical polymers.