Malaria is an infectious disease caused by Plasmodium parasites and transmitted by Anopheles mosquitoes. The ability of a given mosquito to transmit malaria parasites is determined by different factors: mosquito genetic factors (TEP1, responsible for mosquito resistance to malaria parasites), parasite genetic factors (Plasmodium falciparum Pfs47 surface protein, allows P. falciparum to by-pass the mosquito TEP1-dependent antiparasitic response) and diverse environmental factors. To identify novel parasite factors that contribute to Plasmodium infectivity in mosquitoes, the group of Stephanie Blandin has established a mutagenesis screen in the murine parasite Plasmodium berghei. They have recovered mutant parasites that are able to invade a normally fully resistant mosquito line. To identify the causative mutation(s), a virulent parasite VP1 was isolated, cloned and phenotyped (both the virulent parasite and the clones) in naturally resistant mosquitoes. During this project, we optimized the workflow for the phenotyping. We show that the optimal time for an infection that leads to high parasite loads is at 3 and 4 days post passage and that the optimal time for mosquito midgut dissections and microscopic analysis is at 10 to 12 days post infection. We also show that an antibiotic/antimycotic treatment of mosquitoes improves mosquito survival, but at the same time renders resistant mosquitoes more susceptible to infections (with both the wild type and the virulent parasites), indicating that the mosquito microbiota contributes in part to determine the resistance of resistant mosquitoes to parasites. With the intention of distinguishing virulent from avirulent virulent parasite clones, we phenotyped them in naturally resistant mosquitoes. We observed variable prevalences ranging from 2% to 38% for the different clones. Among the 12 clones, one stood out with the highest prevalence (38%) and parasite load (6), consequently being the best candidate for further exploration, despite its potentially partial phenotypic penetrance.