The grass festulolium, a hybrid between the genera Festuca and Lolium, has a variety of beneficial agronomic attributes derived from both parents. Compared with high-ploidy festulolium, diploid festulolium is well suited to stabilizing ploidy and for studying agronomic traits and genetic relationships. We sought to produce a diploid festulolium hybrid that was resistant to summer depression, by hybridizing diploid Lolium multiflorum Lam. and hexaploid Festuca arundinacea Schreb., which has a high tolerance to summer depression. We obtained seven diploid F-4 plants that were capable of surviving the extremely hot summer in Morioka, Japan, in 2010, which was 2.7 degrees C higher than the average summer temperature. The observed resistance to summer depression in these plants was likely due to heat stress tolerance. The genomic constitutions of these seven hybrids were analyzed by GISH, and the chromosomal characteristics of a single diploid F-4 was analyzed by FISH using rDNA probes. The results showed that although no Festuca-specific genome remained in any of the seven diploid F-4 plants, extensive chromosomal rearrangement was observed in one of them. Our findings suggested that hybridizing diploid L. multiflorum and hexaploid F. arundinacea may be useful for modifying chromosome architecture in the Lolium genome with potential applications in chromosome engineering.