We present a novel KING breeding design to produce genetically protected homogeneous fish material for commercial producers from a breeding nucleus. KING F2-production population is established from the nucleus, first through full-sib mating within two unrelated high-quality families to produce inbred F1-progeny and then resolving the inbreeding in F2 through mating of the unrelated F1-individuals. Owing to a small number of founders and the inbred F1-parents, the remaining additive genetic variance is 37.5% of the original. This restricts the use of F2-progeny to establish new breeding programmes, thereby protecting the genetic material of the nucleus. The theoretical calculations show that a concomitant decrease of phenotypic variance is possible. However, the reduction is considerable only for traits with high heritability (h2 > 0.50). The method was tested with rainbow trout. The results revealed that the mean body weight of the KING-progeny was similar, but, surprisingly, phenotypic variation (especially due to residual variance) was higher compared with either their outbred control group or the nucleus breeding population. Although further evaluation of this breeding design is needed, the results suggest that while genetic protection is achieved, the efficiency of the method to reduce phenotypic variation is limited for economically important traits with low-to-moderate heritability.