Described herein are polymorphisms, and in particular single nucleotide polymorphisms (SNP) associated with late onset of sexual maturation in rainbow trout (Oncorhynchus mykiss). In particular, provided are methods for predicting late onset of sexual maturation in rainbow trout, methods for selecting a rainbow trout having late onset of sexual maturation and kit suitable for carrying out said methods. Further provided are rainbow trout cells, sperm and unfertilized eggs carrying at least one allele conferring late onset of sexual maturation in their genome.