Explanation and summary
The fourth plague of the 10 plagues that the God of the Israelites sent as judgment upon Egypt was a swarm of flies. In this plague, swarms of flies cover the land of Egypt. The Egyptian gods insulted by this plague include Hathor (a goddess represented by a cow), and perhaps others as well. The story is recorded in Exodus 8:20-32 (see text below).
As the seasons pass on, after the recession of the waters, the flies become more and more numerous until they are almost a plague every year. The increased severity of this plague, and the providential interference to separate between Israel and the Egyptians, drove Pharaoh and his people to such desperation that Pharaoh gave a half-promise of liberty for Israel to sacrifice "in the land."
This called out the statement that they would sacrifice the "abomination of the Egyptians." This may have referred to the sacrifice of sheep, which were always held in more or less detestation by Egyptians, or it may have had reference to the sacrifice of heifers, the cow being the animal sacred to the goddess Hathor. The new element of separation between the Israelites and the Egyptians introduced into this plague was another step toward establishing the claims of Yahweh to be the God of all the earth and to have taken Israel under His special care.
Biblical Text (KJV)
20 And the Lord said unto Moses, Rise up early in the morning, and stand before Pharaoh; lo, he cometh forth to the water; and say unto him, Thus saith the Lord, Let my people go, that they may serve me.
21 Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are.
22 And I will sever in that day the land of Goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am the Lord in the midst of the earth.
23 And I will put a division between my people and thy people: to morrow shall this sign be.
24 And the Lord did so; and there came a grievous swarm of flies into the house of Pharaoh, and into his servants' houses, and into all the land of Egypt: the land was corrupted by reason of the swarm of flies.
25 And Pharaoh called for Moses and for Aaron, and said, Go ye, sacrifice to your God in the land.
26 And Moses said, It is not meet so to do; for we shall sacrifice the abomination of the Egyptians to the Lord our God: lo, shall we sacrifice the abomination of the Egyptians before their eyes, and will they not stone us?
27 We will go three days' journey into the wilderness, and sacrifice to the Lord our God, as he shall command us.
28 And Pharaoh said, I will let you go, that ye may sacrifice to the Lord your God in the wilderness; only ye shall not go very far away: intreat for me.
29 And Moses said, Behold, I go out from thee, and I will intreat the Lord that the swarms of flies may depart from Pharaoh, from his servants, and from his people, to morrow: but let not Pharaoh deal deceitfully any more in not letting the people go to sacrifice to the Lord.
30 And Moses went out from Pharaoh, and intreated the Lord.
31 And the Lord did according to the word of Moses; and he removed the swarms of flies from Pharaoh, from his servants, and from his people; there remained not one.
32 And Pharaoh hardened his heart at this time also, neither would he let the people go.
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).