Thirty-four recessive photosynthetic mutants of the high-chlorophyll-fluorescence (hcf) phenotype have been isolated by screening 7700 M2 progenies of ethyl methane sulfonate-treated seeds of Arabidopsis thaliana. Most of the mutants isolated were found to be seedling-lethal, but could be grown on sucrose-supplemented media. Chlorophyll (Chl) fluorescence induction, absorption changes in the reaction-centre chlorophyll of PS I (P700) at 830 nm and Chl a/Chl b ratios were recorded in order to probe the photosynthetic functions and to define the mutational lesion. These studies were complemented by immunoblot and Northern analyses which finally led to the classification of the mutants into six different groups. Four classes of mutants were affected in PS I, PS II (two different classes) or the intersystem electron-transport chain, respectively. A fifth mutant class was of pleiotropic nature and the sixth class comprised a Chl b-deficient mutant. Several of the mutants showed severe deficiencies in the levels of subunits of PS I, PS II or the cytochrome b6/f complex. Thus the mutational lesions could be located precisely. Only one mutant was defective in the transcript patterns of some plastid-encoded photosynthesis genes. Hence most of the mutants isolated appear to be affected in translational and post-translational regulatory processes of thylakoid membrane biogenesis or in structural genes encoding constituent subunits of the thylakoid protein complexes.