Verses 20-32 “20And 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.
21Else, 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.
22And 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.
23And I will put a division between my people and thy people: to morrow shall this sign be.
24And 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.
25And Pharaoh called for Moses and for Aaron, and said, Go ye, sacrifice to your God in the land.
26And 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?
27We will go three days’ journey into the wilderness, and sacrifice to the LORD our God, as he shall command us.
28And 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.
29And 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.
30And Moses went out from Pharaoh, and intreated the LORD.
31And 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.
32And Pharaoh hardened his heart at this time also, neither would he let the people go.”
The 4th plague, the flies, no doubt brought disease, maggots, and filth everywhere. Pharaoh was mighty anxious to get rid of the flies. Moses said, the Hebrews could not sacrifice in the Land of Egypt, as Pharaoh suggested, because the Egyptians worshiped the very animals that the Hebrews sacrificed. Such sacrifices in the land of Egypt would lead to rioting and great violence. So, Pharaoh equivocated for the first time about letting the Jews go. This is a testimony to the great power of God to convince a sinner. God can use something as small and weak as a fly to demonstrate His mighty power.