The vibration of a tube array in the entrance region of a high performance steam generator, which is caused by the flow of the coolant fluid, has been shown to be random and having small amplitudes in Part 1 of this series. In this second part, the results of tests with modified tube boundary conditions are presented. Fluidelastic instability leads to chaotic vibration. All of the flexible tubes are at the extremities of the tube array, where the fluid-flow enters or exits the array. Even in this position the tube instability can be predicted by a Connors’ type criterion. Chaotic vibration occurs because the tubes have small gaps at the intermediate supports along the tube-span. A fractal dimensional analysis shows good coincidence with physical explanation of the phenomenon based on observation.