The gill histology in fish is used as an identification of infectious and non-infectious diseases. This approach has also been used in recent years as a biomarker for the determination of pollution and as a tool in environmental monitoring. The gill structure is extremely influenced by external factors, and might serve as an indicator of many problems. In the present study, pathology associated with Ichthyobodo necator, Ichthyophthirius Apiosoma sp., Dactylogyrus sp., Gyrodactylus sp. (parasites), Flavobacterium sp. (bacteria) and concrete contamination were investigated on different fish gills (Oncorhynchus mykiss, Salmo coruhensis, Salvelinus fontinalis, Acipenser baeri and Symphysodon discus). The gill tissues of all examined fish were fixed in 10% neutral buffered formalin embedded in paraffin blocks and stained with hematoxylin and eosin. Hyperplasia and lamellar apposition were observed as the most common pathologies on the gills analyzed. However, abundant necrotic cells, as an indication of a more serious complication, were also observed on the gills of Symphysodon discus infested by Gyrodactylus sp. The vacuolisation of the gills in brook trout, infected with Flavobacterium sp. was recorded as another notable finding.