The availability of purified Bonamia ostreae suspensions and the development of an inoculation protocol enabled experimental infections with the European flat oyster, Ostrea edulis, to be carried out. Different infectious doses were tested and the kinetics of bonamiasis studied. The 50% infectious dose (ID), determined from inoculations with 10 to 106 B. ostreae per individual, was estimated to be 80 000 parasites for 3-year-old oysters. This 50% ID was used to screen for parasite-resistant oysters. Preliminary results suggested that the F1 progeny of oysters that survived previous outbreaks of bonamiasis were more resitant to B. ostreae. © 1995.