NIOSH case studies of worker deaths due to confined space hazards involving wastewater treatment systems were presented. In case one, two workers were asphyxiated in a fire in the floating cover of a digestion tank at a secondary wastewater treatment facility. They had gone into the cover, the only entry and exit of which was a manhole 30 inches in diameter, to heat the interior with propane fueled salamanders in preparation for painting. In case two, two workers and two persons attempting rescue were overcome by gas or drowned when raw wastewater flooded a deep underground pumping station. In case three, two workers were overcome by hydrogen- sulfide (7783064) when they climbed down a ladder into a sludge distribution chamber to retrieve pieces of a broken plastic cover that had fallen into the chamber. The three cases illustrated some of the distinguishing characteristics of confined spaces and some of the important "don'ts" to be considered when workers enter confined spaces. NIOSH recommendations for the most hazardous types of confined spaces (Class "A") were summarized: identification, labeling, and posting of all such spaces; a structured program for personnel training; periodic medical examinations including demonstration of worker's ability to use respirators; entry into a confined space authorized by permit only; prohibition of entry until initial testing from the outside; proper preparation of confined spaces and complete isolation from other systems; specific written procedures for work, observation, and rescue activities; and maintenance of records relating to the recommended procedures.