The basement toilet gurgles and bubbles when the upstairs toilet is flushed. Also the basement toilet bowl does not retain the proper water level, it drains down almost to the bottom of the bowl.

Sounds like a clogged vent stack. The vent is trying to vent itself via the other toilet. Rod the stack from the roof with enough snake to reach roof top to basement floor plus 20'

I will try the vent stack as you suggested. However,we have a septic system. Is it possible that the gurgling and bowl level is due to a back up in the septic tank?

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we have a septic system. Is it possible that the gurgling and bowl level is due to a back up in the septic tank?

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Not very likely. While that might explain the bubbles it wouldn't explain the loss of water in the bowl. Only twe things would cause that.
1. A hidden crack in the bowl, **OR**&
2. Suction from a blocked vent.
I agree with Bob. Check your vents. Good luck, bTom