This paper addresses a challenge: Is a closed-loop system without external excitation identifiable? The so-called fast-sampling direct approach provides a positive answer. It removes a traditional restrictive identifiability condition for linear output feedback closed-loop systems, i.e., an external persistently exciting test signal is not required. Identifiability is analyzed using the lifting technique, the bifrequency map and bispectrum concepts. The proposed approach is further investigated and evaluated by simulation. (C) 2003 Elsevier Ltd. All rights reserved.