AbstractCounts were made of hyphae of the endophyte Acremonium loliae Latch, Christensen & Samuels in leaf sheaths of perennial ryegrass (Lolium perenne L.) cv. Ellett. Material was collected from grazed plots at 4 North Island sites during a 2-year period. Numbers of hyphae/mm breadth of leaf sheath varied by over tenfold in infected tillers collected from the same site at the same time, but mean counts from 40 tillers/collection showed regular and repeatable trends. At 3 sites with yearly rainfalls of 1100–1200 mm, mean counts were similar at the same time, being highest in summer and lowest in winter. At these sites the rise and fall between the 2 extremes was regular. However, at the fourth site, with a yearly rainfall of about 800 mm, counts were high in November and December, were depressed from January to March, reached a peak in April or May, and then declined. A second endophyte was seen in some plants. However, the incidence of each endophyte was generally independent of the other.