Broodstock of the Brazilian flounder Paralichthys orbignyanus (Valenciennes, 1839) kept in the laboratory suffered recurrent heavy infestations by the ectoparasitic dinoflagellate Antyloodinium cf. ocellatum. Between 10 January and 26 February 2003 we monitored A. cf. ocellatum dinospore (infectious motile stage) abundance in a maturation system in order to predict amyloodiniosis outbreaks. Though daily water exchange rate of the tank containing the specimens was 150% of total tank volume (2,500 L), by 15 January the dinospore abundance in the tank reached 1,800 cells/L and on 25 January 7,200 cells/L. There was a subsequent small decrease in dinospore abundance, but by the end of the study period counts were still around 3,000 cells/L. Infested fish were successfully treated with copper sulfate (1.5-mg Cu/L for 24 h during 7d). Observation of the biofilm from the bottom of the tank showed a high number of resting cysts (tomonts) of A. cf. ocellatum after treatment. Apparently, the copper sulfate forced the detachment of the trophonts (feeding parasitic growth stage), and generated the high number of tomonts at the bottom of the tank. The copper sulfate concentration used in the treatment was not effective to kill the tomonts. After a disease outbreak in March 2002 and fish recovery, the biofilm with tomonts at the bottom of the tank was removed by brushing and the use of hydrochloric acid (HC1 30%v/v). After this, no infestation occurred for at least a month. Meanwhile, fish in a nearby tank, where biofilm was not removed, had three amyloodiniosis outbreaks. Our results show that the water exchange rate applied was not sufficient to eliminate the dinospores from the water column, or to remove and eliminate the tomonts from the biofilm. We suggest that cleaning the biofilm of tanks after treatment of infested fish should be considered as a prophylactic measure in order to avoid recurrent amyloodiniosis.