Bivalves from the Great Barrier Reef were examined for Perkinsus infection using the thioglycollate method (Ray, 1966). They were taken from Heron, Lizard, Orpheus, North Direction and Thetford Reefs. Thirty of the 84 species examined were infected (160 out of 644 individuals). They belonged to 23 families. The 4 most frequently infected families, in descending order, were Chamidae, Aridae, Tridacnidae and Spondylidae. All of 4 moribund Tridacna gigas examined were infected.