We have compared the thermal and hydrothermal stability of the acid forms of ZSM-20 and Y zeolites; the catalytic behaviour and structural resistance relative to vanadium poisoning of these catalysts were also investigated. Structural stability was evaluated by X-ray diffraction, and the transformation of n-heptane, performed at 623 K, was used as the catalytic test reaction. The higher thermal stability of H-ZSM-20 relative to HY is clearly confirmed, while its hydrothermal stability is quite similar to that of USHY. The results obtained for the metal poisoning study with H-ZSM-20 were compared, on one hand, to the performance of a non-stabilised acidic form of Y zeolite, HY, and, on the other hand, to its ultrastabilised form, USHY. HY zeolite presented a significantly lower resistance to metal attack than H-ZSM-20. However, H-ZSM-20 suffered a more important metal-induced crystallinity loss than USHY. Nevertheless, the vanadium effect on the initial cracking activity was comparable in both catalysts : H-ZSM-20 behaves analogously to USHY, in particular for the most severe treatment, with no need of any stabilising modification after synthesis.