The temperature dependence of the electrical conductivity and the low-temperature magnetoresistance for iodine-doped highly conducting polyacetylenes are reported. The overall behavior of the temperature dependence is explained in terms of the phenomenological Sheng model, but the selection of parameter values is not unique, suggesting that the model is not sufficient to characterize the samples. The temperature dependence changes rather drastically with the conductivity, which is determined by the doping concentration and chemical reactions within (CH)x. The magnetic-field effect is rather insensitive to the field direction. This indicates that the classical orbital effect is not the principal cause. The magnetoresistance is negative for the sample with the highest conductivity, and its magnitude exceeds by far the upper bound arising from the quantum correction based on weak-localization theory for isotropic media. We evaluate the effect of the anisotropy in the electron diffusivity on the magnitude. With a decrease in conductivity, positive magnetoresistance emerges at low magnetic fields. The drastic variations of the temperature and the magnetic-field dependence with the conductivity show that the highly conducting polyacetylene is close to the metal-nonmetal transition boundary.