Phallostethus cuulong is a species of fish in the family Phallostethidae.

This species is found at depths of 0.5 to 3.5 m (1.6 to 11.5 ft) in tidal canals on the Mekong Delta.

This species attains about 25 mm (0.98 in) in standard length and is translucent white.

Males have their gonads under the chin – along with a serrated saw and rod for grasping females.

[2] The name cuulong is from the Vietnamese name of the river system where they are found, Cửu Long River.