A ground working tool comprises a tooth holder (70) and a replaceable tooth (30), the tooth holder (70) comprising a recess (74) for receiving at least part of a tooth (30), the recess (74) having an upper surface (80) and an opposed lower surface (78), at least one projection (92, 94) of hard material extending from the upper surface or the lower surface of the recess of the tooth holder (74), the tooth (30) comprising a distal part (32) for engaging or working the ground or earth during use and a proximal part (34), the proximal part (34) of the tooth (30) being received within the recess (74) of the tooth holder (70), the proximal part (34) of the tooth having an upper surface (33) and a lower surface (35), the tooth (30) having at least one recess (60, 62) on the upper surface or the lower surface of the proximal part thereof, the at least one recess (60, 62) being of complementary size and shape to the at least one projection (92, 94) of hard material in the recess of the tooth holder such that the at least one projection is positioned within the at least one recess of the tooth when the tooth is fully inserted in the recess of the tooth holder.