The ciliate protozoan Cryptocaryon irritans Brown, 1951, the 'marine white spot', causes one of the most important parasitic fish diseases, with extensive losses every year in mariculture and in the ornamental fish industry. In the present study, we explore the in vitro use of 8 different compounds against the theront (infective) stage of C. irritans; these compounds include extracts of natural products (epigallocatechin gallate (EGCG), L-DOPA, papain), peracetic acid-based compounds (Proxitane 5:23 and 15% peracetic acid, PAA), quinine-based compounds (quinacrine hydrochloride and chloroquine diphosphate) and hydrogen peroxide. All of these compounds had an effect on theront survival; however, only EGCG caused significant theront mortality when applied in doses > or =50 mg l(-1) and over a period of 3 h; papain caused a maximum theront mortality of <50%. We discuss the type of application and potential utility of the compounds tested as part of a management control strategy for C. irritans infections in marine aquaculture and the ornamental fish industry.