【Abstract】 Objective To provide the fundamental data for the control of the flea and to explore insecticidal mechanisms of fenoxycarb to the early third?instar larvae and the newly emerged adults of Monopsyllus anisus (Rothschild, 1907) through histological observations. Methods The early third instar larvae and the newly emerged adults of M.anisus were treated with fenoxycarb by microliter syringes. The changes of tissues were determined by the methods of histology, microphotographics and statistics.  Results The cuticula of the third instar larvae of M.anisus treated with fenoxycarb became thicker, the ovarial rudiments augmented firstly then shrank. The testicular rudiments shrank, and the midgut epithelial cells became atrophy. The testicular plug of the newly emerged adults of M.anisus treated with fenoxycarb disappeared fast, the salivary gland cells were damaged, and the midgut epithelial cells became atrophy. Conclusion （1）Fenoxycarb can cause the death of M.anisus larvae by the disturbance of larvae metamorphosis and changes in cuticula, ovarial rudiments and testicular rudiments or midgut epithelial cells. （2）Fenoxycarb can accelerate the disappearance of testicular plug of the newly emerged male adults. （3）Fenoxycarb can lead to the death of the newly emerged adults of M.anisus by the damage of salivary gland cells and atrophy of midgut epithelial cells.