1. In preparations of about 200 fibres each from thirty-nine biopsies of external intercostal muscle taken from nine myotonic and six nonmyotonic goats, cable properties were determined at 38 degrees C for individual fibres with a pair of intracellular micro-electrodes.2. In each preparation the mean fibre dimensions, determined histologically and corrected for shrinkage, were used to calculate the mean membrane resistance, R(m), fibre capacitance, C(t), and myoplasmic resistivity, R(i). In the 124 nonmyotonic fibres the mean values were: R(m), 1897 Omega.cm(2), C(t), 4.1 muF/cm(2), and R(i), 112 Omega.cm. In 151 myotonic fibres R(m) was 5589 Omega.cm(2), C(t), 4.4 muF/cm(2), and R(i), 103 Omega.cm.3. Conductance of the fibre core times unit length increased with cross-sectional area, and fibre capacitance per unit length increased with perimeter. There was little correlation of membrane resistance per unit length of fibre with either fibre perimeter or resting potential.4. The principal abnormality of cable properties in the myotonic fibre is its threefold higher membrane resistance, which accounts for its decreased electrical current rheobase.