Bonamiasis is a serious disease affecting the oyster industry, particularly in Europe where it has caused catastrophic losses since its appearance in 1979. The causative agent is now recognized as a protistan, Bonamia ostreae(1,2), which aggressively infects Ostrea species to cause clinical disease that rapidly spreads through oyster stocks, usually causing heavy mortalities. The economic importance o f this parasite has generated wide interest but, as David Bucke discusses here, neither its taxonomic status nor its life cycle are clearly understood and there is much to be learn about its pathogenesis and possible means of control.