| Summary: | The Hainan small-toothed sleeper, <i>Microdous chalmersi</i>, is an endemic species disjunctly distributed in Hainan and Guangxi provinces of China. Morphological differences have been previously observed between these populations. We sequenced the mitochondrial genomes of <i>M. chalmersi</i> from Hainan and Guangxi in order to test whether there is a cryptic species. We reconstructed a phylogenetic tree of samples collected from the two populations along with eight representative species of other odontobutids using thirteen mitochondrial coding genes and two rRNA genes. The results showed that five individuals of <i>M. chalmersi</i> from each population clustered into reciprocal monophyletic clades. Furthermore, genetic distance between individuals of the two populations was much larger than that between individuals from the same population. The genetic distance between the two <i>Microdous</i> populations was comparable to interspecific genetic distance of a closely related genus, <i>Odontobutis</i>. We propose that <i>M. chalmersi</i> from Guangxi and Hainan could belong to two different species, but detailed morphological and genetic studies should be carried out to test this hypothesis.
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