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Phytochromes play a key role in the perception of light signals by plants. In this study, the three classical phytochrome action modes, i.e. very‐low‐fluence responses (VLFR), low‐fluence responses (LFR) and high‐irradiance responses (HIR), were genetically dissected using phyAand phyBmutants of Arabidopsis thaliana(respectively lacking phytochrome A or phytochrome B) and a polymorphism between ecotypes Landsberg erectaand Columbia. Seed germination and potentiation of greening, hypocotyl growth inhibition and cotyledon unfolding in etiolated seedlings of the ecotype Landsberg erectashowed biphasic responses to the calculated proportion of active phytochrome established by one light pulse or repeated light pulses. The first phase, i.e. the VLFR, was absent in the phyAmutant, normal in the phyBmutant (both in the Landsberg erectabackground) and severely deficient in Columbia. The second phase, i.e. the LFR, was present in the phyAmutant, deficient in the phyBmutant and normal in Columbia. Under continuous far‐red light, HIR of etiolated seedlings were absent in phyAand normal in phyBand Columbia. The segregation of VLFR in recombinant inbred lines derived from a cross between Landsberg erectaand Columbia was analysed by MAPMAKER/QTL. Two quantitative trait loci, one on chromosome 2 (VLF1) and another on chromosome 5 (VLF2), were identified as responsible for the polymorphism. Phytochrome A is proposed to initiate two transduction pathways, VLFR and HIR, involving different cells and/or different molecular steps. This is the first application of the analysis of quantitative trait loci polymorphic between ecotypes to dissect transduction chains of environmental signals.