many people keep redtail boas in cages that are 4'x2' with no problem, just make sure they have enough hiding spots. My suggestion would be to fill the bottom with several inches of aspen, that way they can hide just about anywhere in the cage they want to. Mine is in a 6'x 2'x2' cage, and my argentine boa is in a 6'x2.5'x2' cage. If you like the boa constrictor look, but worried about the size, you might want to check out the hogg island boa...they do not get nearly as large as their mainland brothers; a male could be housed in a 40 gallon breeder his whole life easily. Another good option is the brazilian rainbow boa...not a very large boa, but their care is alittle more specific. If you get one, just set the cage up first and make sure you have the temps (they tolerate a very narrow range) and the humidity (very very high) exactly where they need it, and they are very simple beyond that!
this is my argentine boa (I know, I've posted this same pic several times before, but it's so cool!)

this is my hogg island boa, she's still pretty young.


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-David
Check out my pet pics at:
http://www.myspace.com/obx_fisherman
1.0 ball python (Pandora - don't ask)
1.0 argentine boa (Prometheus)
0.1 hogg island boa (Andromeda)
0.0.1 brazilian rainbow boa (Inara)
1.0 kenyan sand boa (Diablo)
1.0 normal corn snake(Cypress)
0.1 amery. corn snake (Morgan LaFay)
0.0.2 baby corns (Romulus and Remus)
- 1 normal, 1 ghost
0.0.1 banded cali. kingsnake (Cain)
1.0 tangerine honduran milksnake (Narcissus)
0.0.1 snow corn snake (Valkyrie)
1.0 amazon tree boa (Pegasus)
1.0 colombian boa (Poseidon)
0.1 albino san diego gopher snake (Athena)
0.0.1 sandfish skink (Slick)
0.0.1 fire skink (Phoenix)
1.0 dog (Luke)
the wife's pets:
1.0 bearded dragon (Leonidas)
1.1 ferrets (Ares, Enyo)
0.0.1 betta fish (Captain Morgan)
1.2 cats (Galahad, Ripley and Sassy)
0.1 Boxer