In the Midrash (Shemot Rabbah, Parsha 10, Siman 6), the difficulty of the plague of frogs and the wickedness of Pharaoh after the plague had ceased are described according to the scriptures (Exodus 8:8-11). This is the text: "Moshe went out and cried to Hashem concerning the frogs" – Our Rabbis of blessed memory said, it was not enough for the Egyptians to suffer the destruction of the frogs, but their croaking was even harder on them than the plague itself. The frogs would enter their bodies and croak inside them, as it is said, 'concerning the frogs which He had brought upon Pharaoh' – about the frogs' croaking. "And Hashem did according to the word of Moshe, and the frogs died" – since they had no benefit from the frogs' carcasses and skins, therefore they died. "And they gathered them in heaps upon heaps" – this teaches that each one of them made four heaps, and the land stank, because the Israelites were made to stink from the Egyptians' plagues, measure for measure. "And Pharaoh saw that there was relief" – this is the way of the wicked; when they are in trouble, they cry out, but at the time of relief, they return to their corruption. "As Hashem had spoken" – and where did He speak? "And I know that the king of Egypt will not let you go" (Exodus 3:19), thus ends the quote.

Furthermore, it is stated in the Midrash (Shemot Rabbah, Parsha 15, Siman 27) that the plague of frogs was the hardest plague for the Egyptians: "And the frogs were hard on them, as it is said (Psalms 78:45), 'and frogs which destroyed them,' for they would damage their bodies and castrate them, as it is said (Exodus 7:28), 'and in your bedchamber and upon your bed,' the frogs said to them: the image of your god is nullified and yours remains. Therefore, 'they destroyed them.' And how do we know they spoke? As it is said (Exodus 8:8), 'concerning the frogs.'"