Different semiaromatic polyamides (SAPA) have been synthesized by step-growth polymerization of an aliphatic diamine, M (the 2-methyl 1,5-pentanediamine), and isophthalic acid, I, or terephthalic acid, T, or mixtures of these two diacids. The influence of the relative amount of randomly distributed MT units on the viscoelastic properties of the materials was investigated. It was shown that the glass transition T,, as deduced from DSC thermograms, and the relevant mechanical relaxation T,raise when the content of MT units increases. In contrast, the broad low-temperature secondary relaxation, called gamma, does not markedly depend on the MT content. Samples systematically studied in the absence of any moisture did not exhibit the intermediate-temperature secondary relaxation, called beta, which is characteristic of the wet polyamides. The study of the plastic behavior was focused on the samples MI and I5, which are strictly amorphous, and contain 0% and 50 mol % of MT units, respectively. Mechanical experiments were carried out in both the compression and traction modes, at temperatures ranging from -80 degrees C to T-g. Analysis of the compression data was based on the inspection of the temperature dependence of elastic modulus, E(T), yield stress, sigma(y), plastic flow stress, sigma(pf), and strain softening sigma(y) - sigma(pf). Whereas the plots of sigma(y) as a function of temperature, T, reveal some differences between MI and I5 behavior, a unique master curve was obtained by plotting sigma(y)/E(T) vs. T - T-g, which means that the plastic behavior of these materials is controlled by their chain packing in the glassy state. The strain softening profile of MI and I5 is similar to that already reported in the case of brittle vinyl polymers. This observation is consistent with the traction data, which give evidence for the occurrence of the tensile yielding of MI and I5 at temperatures rather close to T-g.