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Background: Black soldier fly larvae ( Hermetia illucens , BSFL) efficiently bio-convert organic waste into high-value protein, which has significant potential in domesticated animal feed formulations. BSFL growth and bioconversion potential can be enhanced through selective breeding, which requires accurate estimates of genetic parameters and knowledge of genotype-by-diet (G × D) interactions. However, comprehensive knowledge of G × D interactions is limited, and reports of genetic parameters are sparse across genetic strains and production environments globally. Results: This study estimated heritabilities, dominance effects and genetic correlations for BSFL growth traits and quantified G × D interactions. Phenotypes of 2,097 fifth-instar larvae reared on three diets were recorded, including larval body weight (LBW), length (LL), width (LW), and surface area (LSA). All larvae were genotyped using a custom 6K Allegro SNP panel. Genetic parameters and G × D interactions were estimated by fitting an additive-dominance model in ASReml-R. Heritabilities for growth traits were low across diets (0.05-0.14), with diet-specific estimates ranging from low to moderate (0.06-0.36). Dominance effects were significant across the traits (0.09-0.19), and genetic correlations were high among growth traits (>0.81), except between LW and LL (0.51). G × D interactions were moderate across diets (-0.04-0.49). Conclusion: Results suggest that moderate to high genetic gain is achievable over a long-term breeding programme, given the genetic basis of growth traits and BSF′s short generation interval (38-45 days). However, G × D interactions must be considered, either through combined or diet-specific selection strategies, and the significant dominance effects suggest heterosis could accelerate improvement.