Abstract: Apparatus for drilling a horizontal underground bore is adapted for attachment to an articulated arm, or boom, attached to conventional construction equipment such as a crawler excavator. The apparatus includes a hydraulic motor attached to an end of the articulated arm by a support frame which allows for vertical pivoting displacement of the hydraulic motor. A keyed shaft of the hydraulic motor is adapted for secure, fixed attachment to the shaft of an auger. The apparatus further includes a support/guide assembly having a generally flat, plate-like base member and an elongated, linear alignment trough attached to the upper surface of the base member. The base member is positioned on the ground adjacent to where the horizontal bore is to be formed. The trough is aligned along the intended direction of the bore and includes a casing end portion which is inserted into the soil where the bore is to begin and is aligned with the intended direction of the bore.