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A 60-day feeding trial was conducted to evaluate the combined effect of dietary soy phytoestrogens, specifically genistein and daidzein, on the gonadal recrudescence and maturation of male Cyprinus carpio (Linnaeus, 1758). Adult male C. carpio (60 ± 10 g) were fed with a diet with no added genistein or daidzein (C), 110 mg/100 mg genistein (GL), 210 mg/100 g genistein (GH), 4 mg/100 g daidzein (DL), 8 mg/100 g daidzein (DH), combination of 110 mg/100 mg genistein and 4 mg/100 g daidzein (DGL, equivalent to 17.5% soybean meal) and combination of 210 mg/100 g genistein and 8 mg/100 g daidzein (DGH, equivalent to 35% soybean meal) for 60 days. The fish were spent by injection of the inducing agent before the experiment to maintain the homogeneity in maturity stages. Serum 11-ketotestosterone and 17β-estradiol levels increased substantially (p < 0.05), while serum cortisol and 17α,20β dihydroxy progesterone levels were significantly (p < 0.05) lower in GH, DH, DL, DGL and DGH groups. The gene expression of cyp19a1a and 20β-HSD were significantly downregulated (p < 0.05) in GH, DH, and DGH. The ratio of spermatozoa to spermatocytes was lower in high doses of phytoestrogens and their combination treatments (GH, DH and DGH) than in the lower dose fed groups. The GSI (gonad somatic index) values showed a dose-dependent increase in the isoflavone-fed groups. Hence, the study concluded that the feeding of diets with a combination of 210 mg/100 g genistein and 8 mg/100 g daidzein (DGH), which is equivalent to 35% of soybean meal in the diet, disrupted the sex steroid profile and steroidogenic enzymes expression and caused a delay in maturation in the male common carp. Thus, soybean meal can be included in the diet of male common carp broodstock at levels up to 17.5%. However, inclusion levels above 35% have been shown to impair normal reproductive function., Competing Interests: Declarations. Competing interests: The authors declare no competing interests., (© 2025. The Author(s), under exclusive licence to Springer Nature B.V.)