The abrupt transition from low-spatial-frequency laser-induced periodic surface structure (LSFL) to high-spatial-frequency laser-induced periodic surface structure (HSFL) in single-crystal superalloy CMSX-4 during femtosecond laser irradiation has been reported. Microstructural investigations indicate that the transition was initiated by the generation of new grooves on the main ridges of LSFL ripples. This transition resulted in the period of HSFL nearly equal to half of LSFL period. Furthermore, the relationship between both LSFL and HSFL and their parametric dependence was established. The microstructural observation of the abrupt transition provides a morphological evidence of second harmonic generation being responsible for the formation of HSFL. (c) 2010 Elsevier B.V. All rights reserved.