A municipal sewer maintenance worker (victim) drowned inside a sewer wet well. The victim was part of a four-man sewer maintenance crew assigned to clean out a sewer wet well, which was 20 feet deep and 6 feet in diameter. The victim entered through a 24-inch-diameter manway and climbed down on fixed steel rungs which extended to the bottom. The wet well atmosphere had not been tested nor ventilated before entering. The victim was wearing a full-body harness (secured to a winch cable) and a supplied-air respirator. After descending 8 feet to a grating platform, the victim installed an inflatable sewer plug into the end of an 18-inch-diameter inlet sewer pipe 2 feet below ground level. After inflating the sewer plug the victim climbed down to the bottom of the wet well and began cleaning out the wet well with an 8-inch-diameter sewer vacuum hose. Within a few minutes the victim removed the respirator facepiece, complaining to a co-worker that the respirator was in his way. Approximately 30 minutes later, the sewer plug gave way causing sewage to flood the wet well. Co-workers were unable to rescue the victim and he subsequently drowned. The NIOSH investigator concluded that, in order to prevent future similar occurrences, employers and employees must: 1) follow sewer plug manufacturer recommendations on the installation and use of inflatable sewer plugs; 2) develop, implement and enforce specific confined space entry procedures; 3) ensure that appropriate rescue equipment is utilized during confined space entry; 4) ensure that appropriate personal protective equipment is properly worn; 5) consider the feasibility of installing self-priming wet well sewer pumps.