Its a she/male!

[guppymonkey]

I originally got two swordtails, a male and a female but now it looks like my female decided to become a guy! I heard that they can undergo a sex change but it still is surprising. They looked full grown. Its very odd.

 

[gadazobe]

You must have bought juvenilles and his sword tail hadn't grown yet

 

[Thomas A. Anderson]

Either that or you have some really weird chemicals in your water or live near a nuclear reactor.

 

[cheirodon]

Swordtails do it, plattys do it, and some say mollies and guppys do also. And it doesn't have to do with being mature. Sex change can occur, either way, in mature fish. I've only seen females become males, but books state that it can also happan the other way.

Now, what induced the sex change?, that's a totally different question
Do you live near Chernobyl?

...Just kidding!

 

[snotirl]

Ye that can happen u should look and the anal fin down by its bum too see if its male or female!

 

[guppymonkey]

Ye that can happen u should look and the anal fin down by its bum too see if its male or female!

I know about the anal fin to sex swordtails and other livebearers (platies and guppies). It was a nice big fan of an anal fin when I got them and slowly it seemed to be pulled up toward the body and get more rod-like. I wish I had a digital camera to take pictures of the progression.