Describes the application of scanning electron and optic microscopy to diagnose and determine the state of conservation of wooden beams in the 18th century house of San Ignacio in Havana, Cuba. The principe cause of deterioration is fungi. Among the types of fungi identified were: cladosporium nerbarum, aspergillus, pinus, penicillium. Their growth was attributed to high relative humidity and raised temperature. The article recommends three measures: using a fungicide to contain the fungi on the beams; impermeabilizing the surface of the beams; developing the diagnosis of deterioration using electron microscopy. -- ICCROM