Male or Female Koi fish?- How to sex koi

Wondering which of those beautiful koi in your pond are male and which are female? Learning how to sex koi fish isn’t as simple as with some species. With a little knowledge though it isn’t too difficult. There are a handful of traits to look for when determining the sex of your koi.

This can be useful knowledge if you are considering breeding your koi, or you just want to give them appropriate gender names. This knowledge may also come in handy in diagnosing certain koi diseases. 

Sexing Koi Fish

As stated above, there are a variety of ways to sex koi fish. The first thing you should be aware of however is the age of the koi. These specific traits work in identifying koi sex only if the koi are mature. 

A mature koi is one that is approximately 3 years old and just under a foot long. With koi younger and smaller than this it is difficult to determine sex.

Body Shape

One of the first things you can learn to look for when sexing koi is body shape. If you look at your koi from directly above you will soon notice two different body shapes. The koi that have more slender bodies are the male koi. Those koi that look plump or fat are the females. 

The reason for this is rather obvious. Female koi are the ones who have to carry eggs and thus need larger bodies to do so.

As I mentioned above using this feature to determine sex won’t work on immature koi. In fact, with immature koi the opposite is actually true. Immature male koi are often larger than their female counterparts.

Fin Size & Shape

The pectoral fins on a koi are another telltale sign of sex. These are the fins right behind the gills. If you look carefully the pectoral fins of a male will be more pointed and solid than a females. The female pectoral fins will look more rounded in shape. 

This is a little different with butterfly koi fish, however. In these koi, size is the defining factor, not the shape. A male butterfly koi will have longer and thicker pectoral fins while the female’s fins are shorter and thinner. 

Vent Area

The vent is another area of distinction. A females vent will be slightly larger and protrude from the body more than a males will. 


What in the world is a tubercle you may ask? Tubercles are little white spots (not to be confused with Ich) or growths that appear on the head and pectoral fins of a male koi. These form during the breeding season and will go away after.

Females do not develop these so they are a pretty sure sign of a male. 


Outside of winter, koi behavior among males and females is more or less the same most of the year. That is until spawning begins.

As spawning begins you will notice certain koi beginning to chase other koi around the pond. These aggressive fish doing the chasing are the male koi. The ones being chased are, of course, the females. 

Final Thoughts 

I hope this gives you the tools you need to determine the sex of your koi. At the very least you will now know whether to name your koi Jim or Jane!