Allowed 2s22pk − 2s2pk+1 transitions in the F I- B I-like ions of elements with Z ≥ 20 are often observed in spectra of high-temperature (Te ≥ 107 K) laboratory and astrophysical plasmas. Calculations of level populations in these ions are important in the interpretation of measured intensities of these lines for plasma diagnostics. Impurity ion densities and radiated power losses in tokamak plasmas, and the emission measure of astrophysical sources, can be derived from spectroscopic measurements using level population calculations. Certain line-intensity ratios can be used to measure the electron density in astrophysical plasmas; this technique may also be useful in some low-density laboratory plasmas. It is therefore important to experimentally check the level populations in high-Z ions using spectra from a well-diagnosed laboratory plasma, such as that of a tokamak. This has been previously done only for some transitions in Fe XVIII - Fe XXII (Suckewer and Hinnov 1979).