The intensity of resonance line radiation from oxygen, nitrogen, carbon and molybdenum impurities has been measured in the high-field (80 kG), high-density (6 × 1014 cm−3) discharges of the Alcator Tokamak, using a 0.4-m normal-incidence monochromator (300–1300 Å) with its line of sight fixed along a major radius. Total light-impurity concentrations of a few tenths of a percent have been estimated by using both a simple model and a computer code which included Pfirsch-Schlüter impurity diffusion. The resulting values of Zeff, including the contributions due to both the light impurities and molybdenum, were close to one. The power lost through the impurity line radiation from the lower ionization states accounted for ∼10% of the total Ohmic input power at high densities.