Technical Abstract: Ryegrasses (Lolium spp.) are cool-season grasses used for both turf and pasture. Knowledge of genes controlling morphological traits important in seed production would help in understanding relationships among species and assist breeders in plant improvement. Quantitative trait loci (QTLs) for a number of traits associated with seed yield of ryegrass were identified using a perennial (L. perenne L.) x annual (L. multiflorum Lam.) hybrid mapping population. The objective of this research was to identify the genes responsible for the morphological differences between perennial ryegrass and annual ryegrass. The population was studied in the field in three locations for two years. QTLs were identified for days to anthesis, floret number, spikelet number, seed number, spikelet weight, seed weight, spike weight, spike length, and lemma and palea dimensions. Anthesis data was significantly correlated across locations within years, but not across years. Anthesis date was negatively correlated with seed traits. QTLs for anthesis date and seed traits clustered on the map, indicating that the association of earlier flowering from <I>L. multiflorum</I> with high seed yield is at least partially due to linked genes or pleitropy. The majority of the linkage groups contain QTLs for anthesis date and seed traits, reflecting the genetic complexity of these characters in ryegrass.