Technical Abstract: Gray leaf spot (GLS) is a serious fungal disease on the important turf and forage species, perennial ryegrass (Lolium perenne) caused by the rice blast fungus Magnaporthe grisea. Early reports suggest little resistance is present in perennial ryegrass cultivars. However, greenhouse inoculations in our lab using one ryegrass isolate suggests some resistance is segregating in an annual x perennial ryegrass (MFA x MFB) pseudo-F2 population. A well-saturated genetic linkage map has been constructed for this population using RFLP, RAPD, AFLP, and SSR markers, and the segregation seen allows quantitative trait locus (QTL) analysis in this population. Potential QTL for resistance to one ryegrass isolate, detected on at least three linkage groups, will be discussed. To confirm that QTL detected in the current population are still detected in the next generation, a resistant segregant from the MFA x MFB population, MF8, has been crossed with a susceptible perennial clone, L4B-5, from a different genetic background, to form a new pseudo-F2 mapping population segregating for GLS resistance. The goal of this study is to perform QTL analysis in the new population, to evaluate the consistency of location, number, and effect of QTL for gray leaf spot resistance between these two mapping populations, and ultimately to understand the genetic architecture of this important trait. Such understanding will facilitate its utilization in perennial ryegrass breeding programs.