This study aimed to identify the causative of the mass mortality observed in zoeal to post-larval shrimp raised in hatcheries in south Iran. For three consecutive months, samples of nauplii and zoea of Litopenaeus vannamei were collected from an affected hatchery located in the province of Bushehr. Upon culture on marine agar, bacterial colonies that produced white, orange, yellow and red pigments were identified. In the hatcheries in which mass mortality was observed, the water columns of the affected tanks exhibited a red-pink color. Therefore, the bacteria that produced red pigment were selected for further phenotypic characterization using polymerase chain reaction (PCR) and virulence bioassays. Our results indicate that this bacterium belonged the genus Pseudomonas and that it was identical to P. mesophilica and P. anguilliseptica. PCR analysis of this bacterium revealed the production of a 150-bp band that was consistent with the Pseudomonas genus. To determine the pathogenicity of the isolated bacteria in nauplii and post-larvae of L. vannamei, we performed bioassay experiments by bath immersion at 27-28°C. Our results showed that culture of nauplii and post-larvae of L. vannamei with the bacteria at a concentration of 1.5-2.0 × 105 CFU/mL in marine broth resulted in a 100% mortality rate 24-48 h post-challenge. In contrast, there was no mortality in the nauplii and post-larvae that were cultured in the absence of bacteria. Upon pathological examination, we found that the color of the larvae was abnormal and pink, with acute necrosis of the entire body 48 h post-challenge.