Phallostethus cuulong is a species of fish in the family Phallostethidae. It was discovered in 2009 and described in 2012. This species is found at depths of 0.5to in tidal canals on the Mekong Delta. This species attains about 25mm in standard length and is translucent white. Males have their gonads under the chin – along with a serrated saw and rod for grasping females.[1] The name cuulong is from the Vietnamese name of the river system where they are found, Cửu Long River.