Abstract: The present invention is a sewer back-up indicator that is positioned within the hole in the cover of the manhole. The indicator includes a bushing and a rod having a valve at one end and an inflatable balloon at the other. The valve is adapted to receive a gas source which supplies the gas down the length of the rod into the balloon, thus inflating the balloon. When the balloon is in the deflated state, the device can be inserted into position within the hole in the manhole cover and subsequently removed from this position without lifting or removing the cover from the manhole. Connected to the other end of the valve is a flexible member that carries a flag having a legend thereon. When the device is in its up position, the flag and flexible member indicate that fluid within the manhole has risen, thus signaling a potential back-up.