Measurements of wood colour were performed on wood specimens of the European and American walnut (Juglans regia L. and Juglans nigra L.) using the system CIEL*a*b. A laboratory test of natural durability was carried out and the effects of fungal decay of wood were demonstrated microscopically. The fungi used in the experiment were white rot fungi Trametes versicolor and brown rot fungi Gloeophyllum trabeum and Poria vaillantii. The results show that it is possible to distinguish sapwood from heartwood of the European and American walnut through brightness measurements. The 2 species cannot be distinguished using measurements of wood colour; however, it is possible to distinguish them anatomically. American walnut wood contains rhomboid crystals and has heterogeneous ray tissue; European walnut wood does not contain crystals and has homogeneous rays. Tests of natural durability show that the European walnut is less resistant to Trametes versicolor activity and that sapwood and pith are less resistant than heartwood and streaks. At exposure to Gloeophyllum trabeum, the test results for the 2 species were equivalent; moreover, the sapwood and pith areas turned out to be less resistant. At exposure to Poria vaillantii, the American walnut proved to be more resistant than the European walnut. The sapwood and pith of both species are less resistant than heartwood. The results show that the streak area and the heartwood of both species are more resistant than sapwood and pith, and that individual fungi cause different stages of decay. The effects of decay of cell walls due to activity of different fungi were presented with microscopic images.