Was hardened - See Exodus 4:21. Pharaoh probably attributed the exemption of the Israelites to natural causes. They were a pastoral race, well acquainted with all that pertained to the care of cattle; and dwelling in a healthy district probably far more than the rest of Lower Egypt.
Related Commentaries of Exodus 9:7
Exodus 9:7
7 And Pharaoh sent, and, behold, there was not one of the cattle of the Israelites dead. And the heart of Pharaoh was hardened, and he did not let the people go.