This is actually the harmonica riff but the guitar underneath is not nearly as strong in Muddy's version so everyone just plays this, and it works better on the 3 string anyway.
The classic John Lee Hooker riff in a Canned Heat tune from the late '60's. The main thing to get right with this one is your right hand. This is a great tune to practice those Hooker boogies, a shuffle with a s sharp staccato back beat.