The force-displacement records of the indentation of silicon single crystals have been monitored with an ultra-micro indentation system using spherical-tipped diamond indenters. The observations with indenters of different radii varying from nominally 5-20 mu m all exhibited similar behaviour. At low loads, the behaviour was entirely elastic and exhibited complete reversibility. At slightly heavier loads the onset of non-linear behaviour occurred, which, in many cases, appeared to be completely reversible. In all the other cases at this and higher load levels, a "pop-out" event occurred during the unloading in what otherwise appeared to be an elastic unloading. The results are interpreted in terms of the indentation pressure-induced phase transformation from silicon-I to silicon-II on loading and to silicon-III upon unloading.