<p>Stromatoporoids are hypercalcified sponges and were major reef-building and reef-associated organisms in the middle Palaeozoic Era, but their skeletons are poorly preserved and continue to challenge interpretations of their original nature. In view of the very low amount of organic matter in the skeletons, fluorescence (FL) imaging has rarely been applied for the study of stromatoporoid skeletal structure. Since the Silurian rocks on Gotland are well preserved, the stromatoporoids there contain enough organic material to examine the microstructure of the skeletons using FL microscopy. FL highlights the presence of small amounts of organic matter in stromatoporoids by green-coloured light emission. By using this method, it is possible to visualize potentially original or near-original skeletal microstructures, which commonly appear extensively altered under transmitted light, particularly under cross-polarised light. Such alteration results from early-diagenetic neomorphism of stromatoporoid skeletons, in particular the formation of fabric-retentive irregular calcite (FRIC) crystals. FRIC permeates the entire skeleton, including its skeletal elements and cement-filled internal spaces. However, the FRIC formation did not redistribute the organic material in the skeleton, so that FL microscopy enables the visualization of the porosity of the galleries and, in certain genera, also the microporosity within the skeletons (microgalleries), demonstrating that many stromatoporoid skeletons originally had extremely high porosity and thus presumably also very high permeability. A systematic investigation of stromatoporoids from other periods and regions using FL microscopy could therefore not only provide valuable insights into the lifestyles of stromatoporoids, but also contribute to the systematics of this group, which remain problematic.</p>