Phasic and tonic abdominal flexor muscle fibres of two anatomically different muscles in the crayfish Procambarus clarkii or Astacus sp. were investigated using electrophysiological techniques. 1. In phasic muscle, fibres were electrically coupled. They were very tightly packed and could not be isolated. No coupling was observed in tonic fibres where single fibres could easily be isolated and their radii determined with precision to between 75 and 200 micron. 2. The electrical constants of phasic fibres (sarcomere length about 2.5 micron) were determined according to the "infinite" cable theory. The were : lambda = 0.5-1 mm, Ro = 30-40 k omega, Rm = 20-30 omega.cm2, Ri = 2-5 omega.cm and Cm = 3-4 microF/cm2. 3. In tonic fibres (sarcomere length about 8 micron) electrical constants were determined according to the equations for the "short cable". They differed considerably from one fibre to another : lambda ranged between 1.5-12 mm, Ro 0.15-1.6 M omega, Rm 700-1,100 omega.cm2, Ri 25-150 omega.cm and Cm 15-300 microF/cm2. 4. The current voltage relationship was linear in phasic fibres for both outward and inward currents not exceeding 0.75 microA. In tonic fibres there was a delayed rectification in about 95% of fibres; anomalous rectification and generation of spikes were found in about 5% of the fibres. 5. In tonic fibres, a strong hyperpolarizing pulse produced a delayed inward rectification followed by prolonged after-depolarization. The addition of TEA to the bathing medium diminished this rectification and abolished the after-depolarization; therefore, K+ ions seem to be involved in this phenomena.