Extensive and informative plasma measurements have been made with the intense and energetic neutral‐particle beams that are used for fueling and heating magnetically confined, controlled‐fusion experimental plasmas. This diagnostic technique does not perturb the plasma because only the unused transmitted fraction of a neutral beam is detected. Orthogonal arrays of highly collimated detectors of the simple secondary‐electron‐emission type are used in magnetic‐mirror experiments to measure neutral‐beam attenuation (directly related to the plasma line density) along chords through the plasma volume at different radial and axial positions. Data from these arrays yield radial and axial plasma‐density profiles, ion angular distributions at the plasma midplane, estimates of the neutral‐beam input power to the plasma, and information on macroscopic plasma motion. Representative results obtained by applying this useful diagnostic technique to the recently completed tandem‐mirror experiment TMX are included here. This diagnostic method can also be advantageously applied to other controlled‐fusion experiments that use neutral‐beam injection.