Hagrid produces about a hundred Blast-Ended Skrewts for the fourth-year students to study in their Care of Magical Creatures lesson. The newly hatched Skrewts are about six inches long; they look like shell-less lobsters, horribly pale and slimy-looking, and give off a very powerful smell of rotting fish. Contained in crates, the Skrewts crawl over each other, bumping blindly into the sides of the boxes. Every now and then, sparks fly out of the end of a Skrewt and, with a small phut, it is propelled forward several inches
Some of the Blast-Ended Skrewts have stings on them, which Hagrid thinks are the males; the female Skrewts have sucker-like additions on their bellies, which he thinks might be to suck blood. Hagrid isn't sure what the Blast-Ended Skrewts like to eat, and gives the students ant eggs, frog livers and some grass-snake to experiment with.
The Skrewts are placed in boxes in the pumpkin patch by the fourth year students in an attempt to discover if they hibernate or not. The creatures apparently do not hibernate, and do not appreciate being forced into the pillow-lined boxes and nailed in. They rampage around the pumpkin patch, the wreckage of the boxes strewn around them. They are eventually restrained and tied up by Hagrid and some of the students.
An enormous Blast-Ended Skrewt is one of the creatures hidden in the maze for the third task. The Skrewt is ten feet long, and looks more like a giant scorpion than anything. The Skrewt is defeated when Harry hits its fleshy, shell-less underside with the Impediment Jinx.