Mirelurk Kings are the leaders of the Mirelurk species in Fallout 3. You'll tend to find them near most irradiated sources of water, or in caves, underground. Unlike the Mirelurks and Mirelurk Hunters they are not part of the the crab species, they're actually a mutated form of Snapping Turtles. Despite being the strongest of the mire lurk race, many would consider them easier to kill than Mirelurk Hunters because they don't have a protective shell and it's easier to aim at their head.
Mirelurk Kings are on par with Super Mutant Masters in terms of health. They have a sonic scream attack that will cripple the players head if it connects. You can avoid this attack by crippling the Mirelurks King's head first. In close combat, Mirelurk Kings will swipe you with their claws, this attack is actually weaker than a Mirelurk Hunter's charge attack.