The back of a one-piece garment is separated into two parts, upper and lower, by a horizontal opening formed in the small of the back and which is connected at both extremities to vertical slits formed along both sides of the garment. The upper half of the back has fastener members on its lower corners with which it can be secured to the front. The lower half has an elastic belt at its upper edge and fasteners along the slits, which together make the garment fit the wearer well at work or during exercise.