We have used in situ conductivity measurements to investigate the defect evolution and accumulation in ion implanted crystalline Si. Upon irradiation at room temperature with 400 keV Si and 2.7 MeV Pt ions the initial conductivity (4 X 10(-2) Omega(-1) cm(-1)) decreases by about four orders of magnitude to a value of 2 X 10(-6) Omega(-1) cm(-1), characteristic of intrinsic silicon, and then slowly increases at higher fluences. The strong decrease in conductivity, observed at low fluences, can be modelled in terms of dopant compensation produced by deep levels introduced by divacancies and complex defects in the band gap. At higher fluences the conduction is dominated by electron hopping in a buried continuous amorphous layer produced by irradiation. These results are reported and discussed.