We have analyzed by Ag- and CMA3-staining, and rDNA fluorescent in situ hybridization an unusual NOR site polymorphism in a population of Salmo trutta from Mino basin (Northwestern Spain). A multichromosomal Ag-NOR distribution largely different from standard individuals both in the number and position of Ag-NORs, as well as in the pattern of activity, was revealed. Atypical Ag-NORs consist of rDNA genes, as evidenced by rDNA FISH, and are capable of constituting their own nucleolus. In spite of the large number of available NORs per individual, the mean number of Ag-NOR per cell was less than 3 in most individuals, which suggests a regulation mechanism to adjust the ribosomal production. The new rDNA clusters detected seem to be stably integrated, however, some characteristics of the NOR pattern observed suggest that this polymorphism is an unstable phenomenon.