Depends on the age of the BN Pleco, and whether it's a male or female. When they get to around 3 inches, you should be able to tell for sure if it's a male or female, as females only have the whiskers around the lip, but the males have them going from the lip to the forehead as well. I've had my BN's a couple months now, and the whiskers are starting to grow up on his forehead, as well as longer whiskers on his lip near his nose.