Phlio waterfall (PL) in Namtok Phlio National Park in eastern Thailand is well known for the abundance of a stream fish, Neolissochilus soroides (Cyprinidae) for many decades. It had been observed that morphology of N. soroides population at PL appeared to be much thinner than those in nearby populations. To clarify this issue, we sampled individuals of N. soroides from four populations associated with four headwater streams and examined their body profiles using 11 morphological characteristics. Results revealed that some individuals of PL population were significantly thinner than others (p < 0.005). This difference was not influenced by the environment nor genetics, and may be related to health of the fish, which will require further investigation.
