Vibrio harveyi recovered from diseased post-larval Penaeus vannamei produced a thermostable exotoxin, which was lethal to Dublin Bay prawns, Nephrops norvegicus L., when injected intramuscularly. The extracellular products (ECPs) concentrated from tryptone soya broth supplemented with 1% (w/v) sodium chloride or from cellophane overlays on marine 2216E agar with incubation at 15, 22 and 27 °C were toxic, with the lethal dose 50% of the crude ECPs estimated to be 4.4 µg protein prawn-1. Proteolytic, haemolytic and cytotoxic activities were detected, although the occurrence and quantity of these activities were influenced by cultural conditions. The ECPs which had been heated (100 °C for 10 min) or digested with protease K produced the same pathology as crude, untreated ECPs. Western blotting demonstrated that all the ECP preparations contained low molecular weight lipopolysaccharides, which may constitute the lethal toxin of V. harveyi.