Abstract: At present, the shortage of fish meal (FM) resources and the rising price are particularly severe in China. The research, development and application of new protein sources to replace FM are imminent. It should be noted that there is a lack of research on the muscle quality of largemouth bass (Micropterus salmoides) after the replacement. To evaluate the effects of single-cell proteins (SCP) on muscle quality and the expression of related genes in largemouth bass, a control group with 45% FM content was compared with eight experimental diets. These diets, formulated to be isonitrogenous and isolipidic, replaced 50% and 100% of the FM with Methanotroph (Methylococcus capsulatus) bacteria meal (MBM), Clostridium autoethanogenum protein (CAP), Chlorella vulgaris protein (CHP), and Spirulina platensis protein(SPP), and were fed to juvenile bass initial weight(8.00±0.50) g for 60 and 240 days, respectively. The results showed that the muscle crude protein did not significantly differ among the groups during the juvenile stage. However, the groups where 100% of FM was replaced with CHP or SPP exhibited significantly higher muscle crude protein compared to other groups in the adult stage. CAP replacement led to significantly higher muscle crude fat across all replacement ratios. During the juvenile stage, the group with 100% CHP replacement had significantly higher ∑EAA (essential amino acids), while the group with 100% SPP replacement had significantly higher ∑NEAA (non-essential amino acids), ∑DAA (delicious amino acids) and ∑TAA (total amino acids) compared to the other groups. In the adult stage, all replacement ratios of CHP and SPP resulted in significantly higher ∑EAA, ∑NEAA, ∑DAA, and ∑TAA, particularly Val, Met, and Gly under 100% replacement conditions. At the juvenile stage, differences among the groups were generally minor, but the CAP and CHP 100% replacement groups had significantly higher C20:3ω-6 content than the other groups. In the adult stage, all SCP replacement groups exhibited a significant reduction in ∑SFA (saturated fatty acids) and a significant increase in ∑MUFA (monounsaturated fatty acids), ∑PUFA (polyunsaturated fatty acids), ∑ω-3, and ∑ω-6, with the 100% CHP and SPP replacement groups showing significantly higher levels of 8 PUFAs compared to the other groups. During the juvenile stage, the muscle hardness, adhesiveness, cohesiveness, elasticity, chewiness, and shear force did not significantly differ among groups, with no notable differences compared to the control group. In the adult stage, all SCP replacement groups exhibited higher values for these indicators compared to the control group, especially in the muscle hardness, cohesiveness, elasticity, and chewiness in the CHP and SPP replacement groups. The water loss rate did not significantly differ among groups in the juvenile stage, but the CHP and SPP replacement groups showed significantly lower water loss rates compared to other groups in the adult stage. Across different growth stages, all four SCPs upregulated the mRNA expression of Myog, Myod, Myf5, Mrf4, and IGF1 genes while downregulating MSTN gene mRNA expression, particularly in the 100% CHP and SPP replacement groups. At both stages, CHP replacement resulted in significantly higher mRNA expression levels of TOR, 4Ebp1 and S6K1 compared to other groups. The study concluded that replacing FM with SCPs significantly affected muscle quality and related gene expression in largemouth bass, particularly during the adult stage and under 100% replacement conditions. Among the SCPs, CHP and SPP significantly increased muscle crude protein, flavour-enhancing amino acids, and polyunsaturated fatty acids while enhancing muscle water retention capacity. These improvements were achieved by regulating the expression of genes related to muscle growth, promoting protein synthesis and muscle development. This study provides a theoretical basis for the application of novel SCP sources in largemouth bass feed formulations.