Oh ok.
Well, I would first check your temperatures. If the snake is getting a little hissy it could be related to temperatures or it could be related to hunger as well. How often do you handle it? Some snakes it is typical of them to be a little hissy, especially while they are young.
Mexican Black Kingsnakes, or even Kingsnakes in general are actually pretty docile snakes for the most part. I would make sure you aren't spooking it when you first open it's enclosure. My young kingsnakes were a little hissy when first purchased, but after some handling they came around just fine.
Hope this helps,
Chris
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Chris Law
U.A.P.P.E.A.L. (Uniting a Proactive Primate and Exotic Animal League)
Herpetoculture Element Representative