Sutton,-D.C.; Trott,-L.; Reichelt,-J.L.; Lucas,-J.S. Assessment of bacterial pathogenesis in crown-of-thorns starfish Acanthaster planci (L). PROCEEDINGS-OF-THE- SIXTH-INTERNATIONAL-CORAL-REEF-SYMPOSIUM,-TOWNSVILLE,-` AUSTRALIA,-8th-12th-AUGUST-1988.-VOLUME-2:-CONTRIBUTED- PAPERS-MINI-SYMPOSIUM-1-TO-10-14. Choat,-J.H.;Barnes,- 1988. pp. 171-176. Between March 1985 and January 1988, crown-of-thorns starfish Acanthaster planci showing symptoms of pathogenesis were examined for the presence of potentially pathogenic bacteria. The starfish, from flow through or recirculating aquaria, had symptoms including lesion formation, tissue degeneration, loss of turgor and collapsed spines. The progress of disease could be halted by treatment with antibiotics. A range of bacteria were isolated from diseased tissues. These included bacteria normally found associated with healthy starfish, but also included bacteria belonging to the genera Vibrio, Pseudomonas and Moraxella . Vibrio harveyi was commonly isolated and V. tubiashi , a known invertebrate pathogen, was isolated on several occasions. Pathogenicity testing of individual isolates and mixtures of isolates against apparently healthy adult starfish (and for some isolates against larval stages of the starfish) failed to induce disease symptoms. The possibility that these bacteria function as facultative pathogens of stressed animals is discussed.