On December 8, 2000, the National Institute for Occupational Safety and Health (NIOSH) received a request from management officials of Seven Crown Resorts to evaluate carbon monoxide (CO) concentrations associated with the operation of houseboats on Lake Mead. On January 24 - 25, 2001, NIOSH investigators conducted a site visit at Lake Mead to investigate CO concentrations on houseboats located at Echo Bay Marina, Nevada. This letter describes our evaluation methods, findings, and conclusions. The Seven Crown Resorts houseboats evaluated at Echo Bay Marina were designed as pontoon boats with the generator exhaust discharging out to the side. The back of the houseboats did not have slides, and the stairs leading down to the water were located on the opposite side of the boat away from the generator exhaust. Warning CO labels were located on the back of the boat and directly above the generator exhaust discharge. These design features help deter individuals from spending time directly near the generator exhaust discharge area. During our evaluation at Echo Bay Marina the prevailing wind was moving the exhaust gases away from the back of the houseboats and not toward the back decks. Therefore, no extremely high CO concentrations (CO concentrations above the NIOSH IDLH value of 1,200 ppm) were measured on the back of the houseboats. IDLH environments were measured with detector tubes and the emissions analyzer directly near the generator exhaust discharge. Therefore, the possibility of high CO concentrations may exist under environmental conditions that would carry the exhaust gases toward the back of the houseboat. A previous study has documented that the area around the back deck of houseboats can be hazardous under certain environmental conditions (i.e., lack of air movement) when the generator or motors are in operation. Individuals swimming or working in the area directly near the generator exhaust (with the gasoline generator in operation) could be exposed to extremely high CO concentrations resulting in CO poisoning or death within a short period of time. This evaluation was performed in January which is not in the prime operating season for houseboats. Activities at the dock were slow, due to the low number of houseboat rentals. Therefore, personal sampling was not conducted. However, general recommendations are provided to help control potential worker CO exposures. In addition, recommendations are provided to reduce the potential for CO exposure around the generator exhaust and back deck on houseboats.