Abstract: The effects of different dosage of Cefalexin on larval development of Penaeus monodon and changes in the number of heterotrophic bacteria and vibrios in larval rearing ponds were studied. During experimental initial stage, whether control or trials, the number of heterotrophic bacteria and vibrios in pond waters increased. After Cefalexin was used again, the number of heterotrophic bacteria and vibrios in all trails' pond waters were obviously lower than that of the control, and the number of heterotrophic bacteria and vibrios in control's pond waters were obviously raised. The 0.5~1.0 LL-1 dosage of Cefalexin can markedly affect to kill and restrain the growth of heterotrophic bacteria in raising seedling pond waters (P0.05）,but can not obviously restrain the growth of vibiros (P﹥0.05). The 1.5 LL-1 dosage of Cefalexin can significantly affect to kill and restrain the growth of heterotrophic bacteria and vibiros in raising seedling pond waters (P﹤0.01）.When larvae developed to the eighth day of post-larval (P8), the number of heterotrophic bacteria and vibtio in control's pond waters is 6.33105 and 3.55104 CFUmL-1, respectiveiy. It was well above the trails'. The time of larvae developing to P1 is above ten hours ahead of the control's. Survival rate of trials is 3.2%~13.1% higher than the control's.