The neoclassical diffusion of a plasma in traps with a helical magnetic axis is investigated theoretically in the regime where the effective trapped-particle collision frequency s of the order of the frequency of trapped-particle oscillations i between the magnetic mirrors. It is shown that in the case considered a formal analogy with the neoclassical transport theory of axisymmetric systems exists. It allows particle and heat fluxes to be obtained, without special calculations, by a modification of the corresponding expressions for the transport coefficients in axisymmetric systems. Expressions for the transport coefficients in the plateau and banana diffusion regimes are obtained. – Studies of this kind are important for the problem of plasma containment in stellarator-type traps.