An experimental study is performed to determine the performance of a ground source heat pump (GSHP) system in the heating mode in the city of Erzurum, Turkey. The GSHP system using R-134a as refrigerant has a single U-tube ground heat exchanger (GHE) made of polyethylene pipe with a 16mm inside diameter. The GHE was placed in a vertical borehole with 55 in depth and 203.2 mm diameter. The average coefficients of performance (COP) of the GSHP system and heat pump in heating mode are calculated as 2.09 and 2.57, respectively. The heat extraction rate per meter of the borehole is determined as 33.60 Wm(-1). Considering the current gas and electric prices in Erzurum city, the equivalent COP of the GSHP system should be 2.92 for the same energy cost comparing with natural gas. The virgin ground in Erzurum basin has high permeability and low thermal conductivity. In order to improve the thermal efficiency of GHE and thus improve COP of a GSHP in the basin, the borehole should be backfilled with sand as low-cost backfill material and a 1 to 2 m thick surface plug of clay should be inserted. Copyright (C) 2007 John Wiley & Sons, Ltd.