Mature oocysts of Plasmodium falciparum and P. vivax from western Thailand were separated from the midguts of Anopheles dirus by collagenase digestion, and the number of sporozoites contained in each was counted. For 26 P. vivax oocysts, the mean count was 3, 688 (range 1, 954-5, 577) and for 14 P. falciparum, the mean count was 3, 385 (range 1, 359-4, 554); a single P. cynomolgi oocyst contained 7, 521. Counts were not significantly correlated with oocyst density, oocyst age, or identity of the examiner. There may have been strain differences in fecundity, particularly between P. falciparum lines maintained in vitro. Mosquitoes receiving a second, uninfected blood meal seven days after feeding on P. vivax-infected volunteers developed no additional sporozoites per oocyst, but had salivary glands 3.4 times as infected. By calculation, more than 20% of P. vivax sporozoites released from oocysts subsequently invade the salivary glands.