The characteristics of primary and secondary asexual peak parasitaemia during sporozoite induced Plasmodium cynomolgi B infections in 40 rhesus monkeys have been studied. Colony bred Anopheles stephensi were fed on different days on the gametocyte carrying monkeys and the infectivity of mosquitoes as determined by oocyst count on day 8 post-feeding was recorded. Following the day of sporozoite inoculation, the mean prepatent period was 8.58 +/- 0.87 days. The primary asexual peak was attained on day 8.38 +/- 2.27 and the secondary peak on day 15.15 +/- 2.63 after patency. Infectivity rate was 100% in 144 batches of mosquitoes fed three days before to three days after the secondary asexual peak. Judging from the day of patency, the high oocyst count was obtained between day 12-18 after patency. Infective gametocyte peak coincided with the secondary asexual peak parasitaemia, with the result that maximum oocyst number was observed in batches fed on the day of secondary peak or one day prior to or one day after the secondary peak. Infectivity during the period of primary asexual peak was inconsistent.