Transmission of malaria parasites by mosquito vectors is dependent on the successful development of Plasmodium infective forms following ingestion of a blood-meal infected with gametocytes. This process is complex and includes a series of morphological and physiological transformations within the mosquito midgut (gametogenesis, fecundation, and ookinete formation) [16]. We previously observed in in vitro and in vivo tests that 100 μM Shiva-3, a synthetic cecropin, inhibited P. berghei ookinetes development [5], [6] and [7]. The similarity of scorpine to cecropin prompted us to investigate its effect on the same P. berghei sexual stages. Our results showed that scorpine completely inhibited both fecundation and ookinete formation at 50 and 3 μM, respectively ( Fig. 3), and that it had a toxic effect on gametes and on ookinetes at lower concentration than that of Shiva-3. The calculated ED50 were 10 μM for the gamete and 0.7 μM for the ookinete stages of development. These results indicate that scorpine could be a better candidate than Shiva-3, to be incorporated into the genome of genetically engineered malaria-resistant anophelines.