When a radiating source is immersed in a homogeneous plasma of infinite extent; an electroacoustic wave may be excited in addition to the usual electromagnetic wave. The electroacoustic wave becomes a longitudinal plasma wave in the far zone of the source.The case of a Hertzian dipole in a lossless plasma is considered first. The fields of both the electromagnetic (e.m.) and plasma modes excited by the dipole are explicitly obtained, and the e.m. and plasma components of the radiation resistance of a Hertzian dipole are determined. The case of a cylindrical-dipole antenna in a lossless plasma is investigated next. The far-zone fields of both the e.m. and plasma modes are explicitly obtained, and the e.m. and plasma components of the radiation resistance are then derived as functions of the antenna dimension and the plasma parameters. Finally, the case of a Hertzian dipole in a slightly lossy plasma is studied; the effect of collisions in a plasma on the radiation resistance of the immersed antenna is discussed.