Evaluations were conducted by NIOSH at six fish hatcheries in Montana, Arizona, and New Mexico to assess the effects of formaldehyde (50000) exposure on fish hatchery workers. Formalin was used at the hatcheries at concentrations of 1,400 to 1,700 parts per million (ppm) for treating trout and salmon eggs to control infections from Saprolegniaceae fungi. It was also used to prevent parasitic infections in fingerling trout. Two air sampling methods were used to evaluate potential formaldehyde exposures of workers. Exposure to formaldehyde was shown to be below the Occupational Safety and Health Administration permissible exposure limits at each site. However, the American Conference of Governmental Industrial Hygienists' ceiling limit of 0.3ppm was exceeded in one hatchery and was probably exceeded for brief periods in three of the others. Engineering controls in use at two of the hatcheries demonstrated that it was possible to greatly reduce worker exposure during the treatment of fish eggs and fingerlings. These controls also eliminated hazards to the eyes and skin due to accidental spillages or splashes. Several recommendations were offered, including the use of chemical protective clothing, proper cleaning and laundering of such clothing, use of chemical safety goggles, provision of quick drench showers and emergency eye wash facilities, as well as provisions to detect and contain any formaldehyde spills, decontaminate the work area, and to ensure proper waste disposal.