Abstract: A fall arresting ladder safety device for attachment to a fixed ladder side rail. The fall arresting device is to be used in conjunction with a body harness worn by the climber with a lanyard attached at one end to the body harness and the other end attached to the fall arresting device. The fall arresting device includes first and second arms pivotally connected in an X-configuration with each arm having first and second ladder side rail engaging portions disposed on opposite edges of the ladder side rail. A spring operably attached to the first and second arms biases the first and second arms between a fall arresting position and a climbing position. To ascend or descend the ladder, the handle portion of the first arm is pivoted upward to disengage the ladder side rail engaging portions of the first arm so that the device can be rolled up or down the ladder side rail as the climber ascends or descends the ladder.