between the livestock of Israel and that of Egypt,
so that no animal belonging to the Israelites will die.' "
'Tomorrow the LORD will do this in the land.'
and found that not even one of the animals of the Israelites had died.
Exodus 9:1-7 (NIV)
However, none of the animals of the Israelites would die. This in itself would be a sign that the LORD was in control of the plague. Also, He gave Pharaoh further proof by setting a specific time when the plague would occur. In this way, there would be no chance that Pharaoh could think that the death of the cattle was all merely a coincidence.
Apparently Pharaoh did not obey the LORD, and the plague occurred as scheduled, for the text says next that Pharaoh sent his men to investigate the status of the animals of the Israelites. Although all the animals of the Egyptians (which had been left out in the field) had died, not even one of the Israelite animals died.
However, Pharaoh's heart seems to be getting steadily harder. Repeated sin will do that to you. This reminds me of a powerful illustration I saw once in a church youth group meeting. They took the strongest guy in the room, (an athletic and muscular person), and began to wind ordinary household thread around his arms, as if to bind him. At first, he was able to easily snap the thread. However, when they wound it about him a bunch of times, even that flimsy thread was able to restrain him. The point was that although a sinful behavior may at first seem to be small or unimportant, repeated sinful practice will result in the person being bound or enslaved by that sin. Of course, every sin is significant. We can't say, 'I'll just do it this one time.' and expect to remain spiritually strong. But repeated willful sin such as Pharaoh is exhibiting is leading to a place where Pharaoh will eventually be powerless to do otherwise. Sin is very destructive, and (as often happens), Pharaoh's sin will lead to unintended consequences for others around him -- in this case the people of Egypt.