This study was conducted to determine a disease outbreak in 14 day old post-larvae of abalone (Haliotis diversicolor supertexta) which caused mass mortality in July 2010 in Shanwei, China. Twenty-nine bacterial strains were isolated from a sample pool of 10 diseased post-larval abalones on 2216E marine agar plates during a natural outbreak of the disease. Among them, a dominant isolate (referred to as strain 21) was found to be highly virulent to post-larvae in experimental challenge tests, with an LD50 value of 1.0 ×104 colony forming units (CFU) mL-1 on day 3. API 20NE kits and 16S rDNA sequence analysis, identified strain 21 as Oceanomonas doudoroffii. It was susceptible to 10 and moderately susceptible to 1 of the 16 antibiotics examined when antibiotic sensitivities of the bacterium were assayed. Results of this study implicated Oceanomonas doudoroffii strain 21 as a cause of mortalities in post-larval abalone from Shanwei, China.