[en] Resonant frequencies were determined for 100 randomly selected tubes and tubes selected for instrumentation during power operation. The average fundamental resonant frequency of the tubes tested was approximately 65 Hz. No spatial distribution of resonant frequency associated with the OTSG was discerned. Mode shapes were determined for five tube spans bounded by the fifteenth support plate and upper tubesheet and one tube span between the ninth and tenth support plates. The boundary conditions associated with the mode shapes varied from simple support to clamped at both ends. The in-air test results are useful for interpreting test data acquired from the permanently installed instrumentation during power operation at Three Mile Island Unit 2