The magnitude and spectral response of second-harmonic generation (SHG) from 0.4 nm single-walled carbon nanotubes (CNTs) in the channels of AlP O4 -5 zeolite is reported. The second harmonics (SH) was found to be polarized perpendicular to the tube axis and maximized by an excitation polarization at 45° to the tube axis. A SH resonance peak at 2 eV was observed, which corresponds to the lowest-energy excitonic state in chiral (4,2) CNTs. The second-order optical susceptibility χ(2) of the system was determined to be 10-6 esu, which agrees with the large χ(2) predicted for small diameter CNTs. These experimental results suggest that SHG can be used to characterize the symmetry and chirality of CNTs. © 2008 The American Physical Society.