An anaerobic pond and a UASB reactor treating the same domestic sewage under the same environmental conditions were monitored at the wastewater research and technology transfer station in Ginebra, Valle del Cauca region in Colombia. Preliminary results showed removal efficiencies in the UASB of around 66, 78 and 69% for COD, BOD and TSS, respectively. The removal efficiencies in the anaerobic pond for the same parameters were 68, 59 and 73%, respectively. A preliminary cost comparison has shown that construction and operation and maintenance costs of the anaerobic pond are 16 and 38% less than those for the UASB, respectively. Based upon this preliminary study, it would seem that technical features of these two systems (AP and UASBs) in terms of removal efficiencies are not key points to choose one over the other since they perform similarly when treating the same sewage. This is particularly true if both reactors work under optimal conditions. Therefore, a more rational and definite answer to the question – UASBs or anaerobic ponds in warm climates? – must be based on a more rigorous appraisal of social, economic and managerial factors linked to the local context.