Biomass yield is one of the most important agronomic traits in forage crops, and increasing biomass yield is the most important objective of breeding programmes. In a precursor study, a genetic map of Lolium perenne (perennial ryegrass) was constructed based on an inbred-derived F₂ population using AFLP and SSR markers. The aim was to map quantitative trait loci (QTL) for biomass heterosis in the F₂ population. Dry weight biomass QTL were identified on linkage groups (LGs) 2, 3 and 7. This work focuses on the fine mapping of the QTL positions by mapping additional ryegrass SSR markers and rice-sequence derived STS markers. The STS markers will be used to reveal fine synteny for LGs 2, 3 and 7 between ryegrass and rice. The QTL positions were recalculated for dry weight data collected in the field and the greenhouse over two years. In accordance with the preliminary analysis, biomass QTL were localized on LGs 2, 3 and 7 with a LOD score value between 3.6 and 14.1 and an explained variance from 3.8% to 22.5%. The addition of markers permitted a reduction in the length of QTL intervals. The combination of the experimental approach of fine-mapping QTL positions and exploiting synteny relationships to rice will benefit marker assisted selection and the future identification of biomass-related genes.