Serological studies of the population having continuous contact with blood-sucking arthropods on the Pacific Coast of Colombia, showed antibodies against a variety of arthropod-borne viruses. In order to investigate some factors related to this finding, a tropical rain forest area representative of this portion of the coast was selected. We have discussed and analyzed the geographical, physical and climatological characteristics of this region that has an abundance of potential arbovirus reservoirs, i. e., birds and mammals. There are three distinct ecological patterns: mangrove swamps, brackish water swamps and tropical rain forest. There is a variety of arthropods (mosquitoes, tabanids, etc.) in each of these areas. The breeding sites producing the widest variety of mosquito species and numbers of mosquitoes are found in tree holes and bromeliads of the tallest trees. Adult insects were captured during the day and night with human and animal bait at ground level and on platforms at varying heights above the ground. We also used a variety of light traps. Apart from wellknown species, we found some new species and others that had not been recorded previously in Colombia. Several types of arbovirus were isolated from mosquitoes and sandflies.