Saimiri boliviensis monkeys were infected by the intravenous injection of 50 sporozoites of the H strain of Plasmodium knowlesi dissected from the salivary glands of Anopheles dirus mosquitoes; prepatent periods were 11, 12, 13, 13, 13, and 16 days. Sporozoites of P. knowlesi stored frozen for 7 days, 53 days, 20 mo, 7 yr and 7 mo, and 11 yr and 5 mo induced infections in Macaca mulatta monkeys with prepatent periods of 7, 6, 8, 10, and 7 days, respectively. After frozen storage for 11 yr and 5 mo, infections were induced in S. boliviensis with prepatent periods of 10–13 days.