The Afrotropical Varanus niloticus is subdivided in two distinct subspecies which are morphologically easily recognizable from each other. Boehme & Ziegler (1997) concluded that these two forms have markedly surpassed the subspecific level and have to be treated as distinct species, the one typical of savannas (V. niloticus ) and the other of rainforest biota (V. ornatus). These authors also reported on the sympatric occurrence (without hybridization and intergradation) of both the forms in the Niger Delta (southeastern Nigeria), which is a further demonstration of the specific status of these taxa. We studied the ecology of monitor lizards in southeastern Nigeria between September 1996 and July 1998. Including both free-living and dead specimens offered in local bush-meat markets, we determined the taxonomic status of several dozens of monitor lizards, ail were typical omatus. Omatus specimens were observed not only in forest biota, but also in derived savannas and cultivated lands of southeastern Nigeria and of the Lagos state, where the two forms were said to coexist. Thus, as far as southern Nigeria is concerned, the coexistence of these two Varanus “species” is still in doubt. In general, Nigerian omatus proved to be habitat generalists, but they were found mainly in secondary swamp forests and along riverine forests. Nigerian omatus did not show any aestivation phase, contrary to niloticus from other African countries. There was evidence of a strong reproductive seasonality in these lizards. Main food of Nigerian ornatus were crabs, a very abundant food resource in the environment. About 10 % of the total number of prey items were vertebrates. Food niche breadth of adults was wider than that of juveniles. Some general implications of the presented ecological data for the evolutionary history of Varanus niloticus species complex are discussed. In addition, some data on the relationships between monitor lizards and local human populations are presented and discussed.