An account of a mold invasion of the Denver Public Library in 1984, caused by a malfunctioning cooling system, and the steps taken to bring this problem under control. About 100,000 items had visible mold. The names of experts and contracting services consulted are listed. Information is given regarding background facts, procedures, and postmortem evaluation. The principal species was <i>Aspergillus niger.</i> The affected area was fogged with ortho-phenyl phenol in 1, 1, 1-trichloroethane. As a result of the mold attack, a new environmental control system is being installed, which has a dual filtration system that includes a 70 percent high efficiency bag filter and an automatic monitoring system. -- AATA