This publication is a revised edition of the NIOSH document Histoplasmosis: Protecting Workers at Risk, which was originally published in September 1997. The updated information in this publication will help readers understand what histoplasmosis is and recognize activities that may expose workers to the disease-causing fungus Histoplasma capsulatum. The publication also informs readers about methods they can use to protect themselves and others from exposure. Outbreaks of histoplasmosis have shared similar circumstances: People who did not know the health risks of breathing in the spores of H. capsulatum became ill and sometimes caused others nearby to become ill when they disturbed contaminated soil or accumulations of bird or bat manure. Because they were unaware of the hazard, they did not take protective measures that could have prevented illness. This publication will help prevent such exposures by serving as a guide for safety and health professionals, environmental consultants, supervisors, and others responsible for the safety and health of those working near material contaminated with H. capsulatum. Activities that pose a health risk to workers at these sites include disturbance of soil at an active or inactive bird roost or poultry house, excavation in regions where this fungus is endemic, and removal of bat or bird manure from buildings. Local, State, and national public health professionals may also find this publication useful for understanding the health risks of exposure to H. capsulatum so that they can provide guidance about work practices and personal protective equipment.