National Institute for Occupational Safety and Health (NIOSH) researchers evaluated two exhaust stack designs for reducing carbon monoxide (CO) exposures from gasoline-powered generator exhaust on houseboats. Tests were conducted (a) after dark, (b) in high-temperature and high-humidity environments, (c) during temperature inversions, (d) under various generator loads, and (e) at different houseboat trim angles. Two different designs of houseboat exhaust stacks were evaluated and compared with the side-exhaust configuration, which is standard on many houseboats. The two designs were flagpole and vertical stack. Both exhaust stacks performed dramatically better than the standard water level, side-exhaust configuration. The highest mean CO concentrations on the upper and lower decks of the houseboat with the vertical exhaust stack were 27 ppm and 17 ppm. The highest mean CO concentrations on the upper and lower decks of the houseboat with the modified flagpole stack were 5 ppm and 2 ppm. These findings are much lower than the 67 ppm and 341 ppm for the highest mean CO concentrations found on the upper and lower decks of houseboats having the usual side-exhausted configuration. The NIOSH evaluation also indicated that high-temperature and high-humidity levels, temperature inversions, generator loading, and houseboat trim angles had little effect on the exhaust stack performance. It also demonstrated the importance of proper design and installation of exhaust stacks to ensure that all exhaust gases are released through the stack. Based on the results of this work, NIOSH investigators continue to recommend that houseboat manufacturers, rental companies, and owners retrofit their gasoline-powered generators with exhaust stacks to reduce the hazard of CO poisoning and death to individuals on or near the houseboat.