The plasma electron density, shape and instability of the 23 kJ θ-pinch have been studied through the use of a Mach-Zehnder interferometer with a giant pulsed ruby laser illumination. The axial interferograms in the range of 45-120 m Torr deuterium pressures gave two-dimensional electron density distributions for the dicharge and established peak plasma electron densities in the range of 6-1016-2.3·1017 cm-3 for the plasma at peak pinch. The time of the peak pinch was about 1/3 of the first half-cycle of the main compressive magnetic field. The plasma characteristic time of containment (time required for the number of particles to be reduced to 1/e) close to peak pinch was 1.8-4.3μs. The interferograms with reverse bias magnetic field showed that the bias magnetic field was trapped in the plasma, and revealed that radial hydromagnetic oscillation of the plasma occurred between the trapped magnetic field and the main compressive magnetic field. The interferograms also showed that the plasma is essentially stable during the first half-cycle of the main compressive magnetic field in the range 45-80 m Torr deuterium pressures.