The anatomical structure of gonads in coregonids is characteristic for teleost fishes with follicular, synchronous group tape-like ovaries and acinose type, lobulated testes. In some alpine lakes in Switzerland and in water bodies in northern Poland, certain disorders and changes in the macroscopic structure of whitefish Coregonus sp. gonads have been observed in the pre-spawning period. In males, the most frequently observed alteration was the division of testes into a larger cranial lobe and a smaller caudal one connected by the spermatic duct, although this has been described as a normality for the species. However, some evident anomalies in the morphological structure of ovaries and testes have been noticed, such as adherence or adhesion of lobes to the peritoneal wall and lateral muscles, asymmetry in the size of lobes, their atrophy or deep narrowing of both gonads. There have also been noted cases of hermaphroditism among both fish fry and adult coregonids. The reasons why such anomalies appear remain unclear, although it has been documented that the formation and deformities of fish gonads can be strongly affected by environmental factors, chemical substances which produce crypto-endocrine effect, pathogens and parasites. The aforementioned changes may have also been an effect of interspecies hybridization, both natural and artificial, or a result of irresponsible actions undertaken by person involve in fishery practice.