Male tilapia were exposed to sub-lethal methomyl concentrations of 0, 0.2, 2, 20 or 200 μg/L for 30 d, and were subsequently cultured in methomyl-free water for 18 d. Relative transcript abundance of steroidogenic genes involved in the HPGL axis of male tilapia was examined at 30 d in the exposure test and at 18 d in the recovery test. The results revealed that low concentrations of methomyl (0.2 and 2 μg/L) did not cause significant changes in gene mRNA levels in the HPGL axis of male tilapia; thus, we considered 2 μg/L concentrations as the level that showed no apparent adverse endocrine disruption effects. However, higher concentrations of methomyl (20 and 200 μg/L) disrupted the endocrine system and caused significant increase in the levels of GnRH2, GnRH3, ERα, and ERβ genes in the hypothalamus, GnRHR and FSHβ genes in the pituitary, CYP19a, FSHR, and ERα genes in the testis, and VTG and ERα genes in the liver, and significantly decreased the levels of LHR, StAR, 3β-HSD, and ARα genes in the testis and LHβ gene in the pituitary, leading to changes in sex steroid hormone and vitellogenin levels in the serum and ultimately resulting in reproductive dysfunction in male tilapia. The recovery tests showed that the toxicity effect caused by 20 μg/L methomyl was reversible; however, the toxicity effect at 200 μg/L of methomyl was irreversible after 18 d. Therefore, we concluded that 200 μg/L was the threshold concentration for methomyl-induced irreversible endocrine disruption in male tilapia.