Several mutants with altered phytochrome A (phyA) signalling have been identified in screenings under continuous far-red light (FR). The latter protocol could preclude the identification of mutants affected in the signalling pathway that operates even under transient phyA activation, compared to the high-irradiance response (HIR) pathway that requires continuous FR. Since some photomorphogenic mutants show shoot-height phenotypes, the screening was conducted on dwarf mutants of Arabidopsis thaliana (L.) Heynh. from the ABRC stocks grown under hourly FR pulses. The dwarf mutant cp3 (compacta 3) showed normal hypocotyl length and folded cotyledons in darkness but enhanced hypocotyl-growth inhibition and cotyledon unfolding under pulsed FR. The HIR and the response mediated by phyB were not affected. Under pulsed FR, seed germination and blocking of greening upon transfer to white light were enhanced in cp3. PHYA levels were normal in cp3. The phenotype under pulsed FR but not the adult phenotype required phyA. We propose that CP3 is involved in the negative regulation of the signalling pathway that saturates with transient activation of phyA.