Throughout its range of South and Central America, visceral leishmaniasis due to Leishmania chagasi is transmitted by Lutzomyia longipalpis. Recently, a new vector, Lutzomyia evansi, has been discovered transmitting the parasite in the Caribbean Coast of Colombia. Field studies, using both experimental and observational methodologies were employed to elucidate the main ecological and behavioural factors affecting disease transmission in the focus of San Andres de Sotavento, northern Colombia. Nine species of Lutzomyia were present and Lu. evansi constituted 90% of all sandflies caught. Flies were most abundant in April, May June and September. Trapping in and around houses showed Lu. evansi to be endophilic but with exophagic behaviour, preferring houses near to forest edge as resting places. Host preference, measured using a newly designed trap in a rotational experimental design, showed that humans were preferred over dogs or opossums (reservoirs) during the peak abundance of Lu. evansi. This was supported by catches on tethered hosts and bloodmeal analysis although location of capture of resting flies was also a significant factor. Mark-release-recapture studies showed that Lu. evansi can move up to 800m after 5 days and that freshly fed flies move a few hundred metres to resting sites. Basic life history data on Lu. evansi was obtained from laboratory rearing. This species was bred under laboratory conditions though high mortalities were seen in first instars. In adults survival was associated with different types of sugar. Flagellate parasites resembling L. chagasi were found in 3 of 5326 wild caught Lu. evansi (0.05%) however, culturing and subsequent characterization of these isolates failed. Experimental infections with L. chagasi showed that at least one strain of the parasite grew more prolifically in Lu. longipalpis than in Lu. evansi. This, together with a limited vector range compared to the Old World L. infantum is suggested to be the result of a recent parasite-vector association. Morphologically no differences were seen between Colombian, Venezuelan and Costa Rican Lu. evansi populations. Some variation was seen however in one enzyme (6GPDH) of 18 isozymes tested. Mitochondrial DNA variation was seen between Central and South American populations.