Third-order nonlinear optical susceptibilities&khgr;(3)oftrans-dihydroxy-1,6,11,16-tetra-(tert.butyl) porphyrazine germanium[t-Bu4PazGe(OH)2]thin films measured by the third-harmonic generation technique are reported for the first time. The&khgr;(3)(−3&ohgr;;&ohgr;,&ohgr;,&ohgr;)value as high as1.3×10−11esu was observed from a dominant three-photon resonance. The&khgr;(3)values of porphyrazine germanium derivative are analogous to metallophthalocyanines and these results imply that axial substitution is beneficial for enhancing third-order optical nonlinearity. ©1997 American Institute of Physics.