Figure 1. Comparison of the relative developmental success of 11 different isolates of Plasmodium vivax in Anopheles albimanus and An. freeborni. The black bars and arrows represent An. albimanus, and the grey bars and arrow represent An. freeborni. The origin of each isolate is indicated on the map. American Type Culture collection reference numbers are as follows: N. Korea T1206, Thai K1294, Vietnam C30151, New Guinea C30060, Nicaragua 30073, Panama C30138, and El Salvador-1 C30197. W. Pakistan, Salvador-2, and Colombia came directly from the Centers for Disease Control and Prevention (CDC). Analysis of feeding and infectivity are as described (4). *Susceptibility of An. albimanus to different stains of P. vivax was tested by feeding the mosquito on infected Aotus monkeys (4). A total of six strains of An. albimanus collected from Panama, El Salvador, Colombia, and Haiti were used to test each of 11 strains of P. vivax, 6 strains of which were from the New World, including 2 isolates from El Salvador and 1 each from Colombia, Haiti, Panama, and Nicaragua; 5 strains were from the Old World, including Chesson strain (New Guinea), West Pakistan strain (Pakistan), North Korea strain (Korea), Pakchong strain (Thailand), and Vietnam II strain (Vietnam). The infection rate was determined by dissecting and counting midgut oocysts or salivary glands during the second week after the infectious blood meal. An. freeborni, which were originally from Marysville, California, and have been maintained in the laboratory at CDC since 1944, were used as a positive control because this laboratory-selected colony has a high level of susceptibility to all strains of P. vivax.