An isotope effect for the hydrogen atom was observed in the highly anisotropic light emission which we newly observed from the laser breakdown plasma of polystyrene microparticles in water. This anisotropic emission had been considered as a hydrogen atomic emission, although its wavelength randomly shifted and the maximum shift was about 2 nm, which was anomalous for atomic emission. To confirm that this emission was a hydrogen atomic emission, we synthesized deuterium-substituted polystyrene ultrafine particles in which the numbers of deuterium atoms were equal to those of hydrogen. The particles were dispersed in water, and their laser breakdown was induced. Consequently, in the anisotropic emission from the underwater breakdown plasma, line broadening which was considered as a superimposition of deuterium and hydrogen Balmer-alpha lines was observed, and the line broadening occurred at every shifted emission. Therefore, the anisotropic emission reverted to the atomic emission of hydrogen which had originated in the particles.