Nonlinear tests performed by using surface plasmon spectroscopy (SPS) on ultrathin films (9-50nm thick) of polyDCHD-HS are reported. PolyDCHD-HS is a new soluble polycarbazolyldiacetylene which seems to show a certain degree of supramolecular organisation even in spin coated films. The SPS tests, which were performed at 1064nm with picosecond pulses, gave a large and negative value for the real χ ( 3 ) susceptibility: |Re{χ ( 3 ) }|=<1.2x10 - 1 7 m 2 /V 2 (significantly larger than what was found in the case of PTS). If further investigations at 1300 and 1500nm confirm these results, polyDCHD-HS is promising for future applications.