Arcellacean (thecamoebian) assemblages recovered from Lake Sentani, a large tropical lake southwest of Jayapura, Irian Jaya, Indonesia, are characterized by low diversity and low abundances. Dominated by Centropyxis aculeata and Arcella vulgaris, this fauna is similar to those indicative of stressed environments (brackish conditions, high levels of industrial contaminants) in temperate regions. However, neither condition exists in Lake Sentani. Previous work has determined that the lake is oligomictic, characterized by weak circulation with turnover occurring only every few years. Prolonged isolation of the lake bottom produces progressively reduced oxygen levels and results in reduced productivity among benthic organisms. The feeble stratification that exists here creates reduced oxygen levels at depth providing a likely explanation of the stressed arcellacean fauna. The oligomictic conditions observed here and the resultant fauna are widespread and are characteristic of a large proportion of tropical lakes around the world. As the low bottom water oxygen conditions will have a serious impact on most benthic organisms in these lakes, other limnological signals including anthropogenic contamination will be masked. This is a disappointing result as the utility that has been developed for the group as a limnological indicator in temperate lakes does not appear to apply in a significant proportion of low latitude lakes.