20 vaccinees (10 from each 50 µg vaccine group) will undergo primary sporozoite challenge 14-30 days after dose 3 via bite of 5 malaria-infected mosquitoes. All subjects will have a blood slide prepared and read to check for asexual P. falciparum parasitemia at least once daily beginning day 5 post challenge. Beginning on day 10 post challenge, subjects will check into a designated hotel, where 24 hour evaluation and care will be available for 10 nights. After this hotel phase, there will be follow-up visits to ensure there are no late developments of malaria in those who have not fallen ill (and thus are considered protected).