Introgression breeding represents a useful method for generating new germplasm resources which combine positive attributes from different species. Within forage grasses, the interfertility of Lolium and Festuca spp. represents a viable route for achieving this end. In order to evaluate the architecture of trait inheritance within this introgression context, a complete chromosome introgression series of F. pratensis (Fp; meadow fescue) in the L. perenne (Lp; perennial ryegrass) back-ground, and derived progeny, were evaluated for shoot and root biomass and N and P uptake in a hydroponic experiment. The results indicated that the Fp parent and the monosomic introgressions relating to Fp chromosomes 3 and 4 showed enhanced shoot and root biomass. However, there was no clear relationship be-tween these phenotypes of the Fp chromosome 3 and 4 monosomic introgressions and that of their derived, respective, progenies. Thus, the interaction between the Lp and Fp genomes in determining phenotype is likely to be complex.