Field studies on the bionomics of adult Anopheles pseudopunctipennis Theobald were conducted to assess its relative importance as a primary vector of vivax malaria in southern Mexico. In four malaria endemic villages in a foothill region near Tapachula, Mexico, population densities of A. pseudopunctipennis increased during the dry seasons of 1990 and 1991. The pattern of nocturnal host-seeking activity indoors was unimodal with a late night peak at 0100 hours enhancing its vectorial significance, because it occurred when most residents were asleep and fully exposed to the anophelines. Comparisons of trapping methods showed that a horse-baited trap was more effective than human landing catches or UV light traps. Pit shelters, on the other hand, were more effective than indoor and natural shelter resting collections. Results of enzyme-linked immunosorbent assays performed on wild-caught A. pseudopunctipennis specimens documented the presence of natural infections with the VK210 and new VK247 circumsporozoite polymorphs of P. vivax. These findings verify the importance of A. pseudopunctipennis as a major vector of vivax malaria at higher elevations and extend the geographical range of the VK247 P. vivax polymorph in Mexico..