Outbreaks of serious mortality among cultured abalone postlarvae have occurred across Southern China since July 2002. Five motile bacterial strains were isolated from diseased abalone postlarvae on tryptic soy agar supplemented with 1% NaCl (TSA1) and/ or thiosulphate citrate bile salt (TCBS) sucrose agar plates during an outbreak in August 2003 in Shanwei, Guangdong province. All isolates were characterized and identi¢ed asVibrio alginolyticus on the basis of biochemical characteristics and comparisonswiththose of the reference strainV. alginolyticus ATCC 17749. Strain 19 (a representative of ¢ve similar isolates) was virulent to abalone postlarvae with an LD50 value of 1.00 104 colony-forming unitsmL 1. All abalone postlarvae exhibited the same signs as in natural outbreaks.The same bacterium could be re-isolated from abalone postlarvae after bacterial challenge usingTSA1andTCBS plates. The results reveal thatV. alginolyticus is an infectious agent of abalone postlarvae.