Intact cells of herbicide-resistant mutants of Chlamydomonas reinhardtii, altered in single amino acids in the D1 protein, were analyzed for chlorophyll a (Chl a) fluorescence decay in terms of electron transfer at the QAQB complex of Photosystem II. The D1 mutants Ar-207 (F255Y). Br-202 (L275F) and DΓ18 (V2191) had an almost unimpaired forward electron transfer (QA⁻QB⁽⁻⁾ → QAQB⁻⁽⁼⁾) and an almost unchanged ratio (0.1-0.2) of the slow to the fast PSII centers, compared to that of the wild type. Thus, the QB binding pocket was minimally modified. However, the mutants Ar-204 (G256D) and DCMU-4 (S264A) had an altered forward electron transfer, an altered apparent equilibrium for QA⁻QB ⇌ QAQB⁻ reaction, and an abnormally high ratio (0.6-0.8) of the slow to the fast PSII centers. These results suggest that these to mutations modified the QB binding pocket significantly. Addition of 100 or 200 mM formate led to an increase in the lifetime of the forward (QA⁻ to QB) electron transfer in the wild type, and the five mutants, with the largest change in the S264A, and the smallest in the L275F mutant. Formate treatment also led to a decrease in the apparent equilibrium for QA⁻QB ⇋ QAQB⁻ reaction and an increase in the fraction of the slow centers. All effects were reversed by 20 mM bicarbonate. We suggest that the amino acid S264, but not L275, F255 and V219, plays an important role in the binding and the function of plastoquinone and bicarbonate in PSII. The role of G256 remains to be further examined since glycine was changed to a significantly different amino acid, aspartic acid.