A survey was made of 64 dairy herds, grazing pastures containing naturalised tall fescue in the North Auckland peninsula, to demonstrate an assooation of endophyte fungus in tall fescue with IBH. Of these, 40 farms had cattle with IBH and 24 had no IBH. Endophyte fungus was identtfied in tall fescue samples from all survey farms. Acremonium coenophiolum was demonstrated in samples from almost all farms and a high proportion of these also showed a Phiafophora-like endophyte. No correlation could be shown between the presence of endophyte or any farm management practices such as topdressing etc and IBH. Breed susceptibility was observed in Friesian breeds whilst Jersey cattle were less susceptible. IBH first appeared in November and December with a peak m January. Most affected herds had a hlstory of IBH for many years. Keywords: Dairy cattle, tall fescue toxicity, endophyte fungus, idiopathic bovine hyperthermia, IBH. heat-stress, hyperthermia.
