Growth hormone releasing hormone (GHRH) regulates the secretion of growth hormone (GH) in the pituitary gland. A 66-bp deletion (c.-923_-858del) was detected in the 5′-flanking sequence of the largemouth bass (Micropterus salmoides) GHRH gene. In two cultured random populations of adult individuals (A: n = 170 and B: n = 150), the genotype ratios of +/+:+/− were 2.5:1 and 2.8:1 respectively. Only one −/− fish was detected. A Largemouth bass family was constructed with two heterozygous individuals (+/−) as parents. The genotype ratio of +/+:+/−:−/− in the filial generation embryos was 1:1.6:0.1 at the neurula and 1:2:0 at hatched larvae stages. This indicated that the 66-bp deletion was a recessive lethal site and that homozygous individuals (−/−) died off in embryonic development. The growth traits (body weight, body length and body depth) were measured, and the GHRH mRNA expression levels in brain tissue were detected using real-time PCR. The effects of genotype (+/−) on growth traits and GHRH mRNA expression were not significant. Although the cause of death was not clear, the results hint that the 66-bp deletion site in GHRH 5′-flanking sequence significantly affects the livability in largemouth bass embryonic development.