After 21 days’ exposure of adult male goldfishes(Carassius auratus) to monocrotophos(0.01、0.10、1.00 mg•L-1), we measured activities of lactate dehydrogenase(LDH), acid phosphatase(ACP), alkaline phosphatase(ALP) in testis and observed micro-, ultrastructure of testis by light and electron microscopy separately. We found that the gonado-somatic(GSI) decreased significantly, ground membrane dissolved, leydig’s cell swelled and interstitial tissue enlarged, as a result of exposure to monocrotophos at the concentration of 1.00 mg•L-1. Monocrotophos induced dissolution of nuclear membrane of the Sertoli’s cell, and the increase of lipid droplet and myelin-like figure number in Sertoli’s cell. The activities of LDH, ACP and ALP in testis decreased gradually, as the exposure concentration of monocrotophos increased. Exposure to monocrotophos at concentrations of 0.10 and 1.00 mg•L-1 resulted in the decrease of LDH and ACP activities in testis. However, the ALP activity did not decrease significantly at the exposure concentrations of all. It is supposed that the energy metabolism was interrupted as a result of the changes of micro-, ultrastructure and the decrease of LDH, ACP, ALP activities in goldfish testis, which caused the reproductive toxicity effect.