Temperature and seasonal changes affected endophyte concentration, peramine concentration, and antixenosis to greenbug in perennial ryegrass infected withAcremonium loliiLatch, Christensen&Samuels. Endophyte and peramine concentrations and degree of greenbug antixenosis to endophyte-infected tillers were lower at 7 and 28%C compared with 14 and 21%C in three genotypes of perennial ryegrass; antixenosis was related to peramine and to endophyte concentration. Genotype differences were greater than differences associated with temperature. Field data collected every 3–6 wk indicated highest endophyte concentrations in spring and fall, when mean daily temperatures averaged 15%C in the previous 4 wk. These data suggest that it is possible to produce cultivars with low or high endophyte concentrations as needed for pastures or turf.