Studies were done on delivery of Plasmodium yoelii sporozoites by Anopheles stephensi into the skin of BALB/C mice. When infected mosquitoes fed on a portion of the ear, 81% of these positive control mice developed parasitemia. When the fed-upon site was excised immediately or 5 min postfeeding, a highly significant, smaller percentage of these experimental mice developed parasitemia. When the delay in removal of mosquito-bitten tissue was extended to 15 min, no significant difference was found between this group and positive control mice. These findings show that mosquito-injected sporozoites tend to remain at the bite site for at least 5 min after the mosquito bite. By approximately 15 min, the first wave of migrating sporozoites has left the bite site and moved into the general circulation. These findings have implications concerning possible host obstruction of sporozoite migration in skin by either anti-sporozoite antibodies or by a cutaneous hypersensitivity reaction to mosquito bite.