Plants modulate their growth and development according to the prevailing light conditions. To detect light signals plants have an array of photoreceptors including the phytochromes which monitor the red and far-red light regions of the light spectrum. Phytochromes regulate gene expression in response to light in part by physically interacting with nuclear-localized bHLH transcription factors called PHYTOCHROME INTERACTING FACTORS (PIFs). PIFs are known to function as negative regulators of photomorphogenesis. Here we show that PIF1, the PIF family member with the highest affinity for phys, is degraded after pulses or continuous red, far-red or blue light in a phytochrome dependent manner. In etiolated seedlings, phyA plays a dominant role in regulating the degradation of PIF1 after a pulse of red, far-red or blue light; while phyB, phyD and other phys also influence PIF1 degradation after prolonged illumination. PIF1 interacted with phyA and phyB in a blue light-dependent manner, and the interactions with phys are necessary for the light-induced degradation of PIF1. In response to red, far-red or blue light treatments PIF1 is rapidly phosphorylated, poly-ubiquitinated and degraded via the ubiquitin/26S proteasomal pathway. In addition, we show that PIF1 negatively regulates photomorphogenesis at the seedling stage. The overexpression of a light-stable truncated form of PIF1 causes constitutively photomorphogenic phenotypes in the dark. pif1 seedlings displayed more open cotyledons and slightly reduced hypocotyl length compared to wild type under diurnal (12h light/12h dark) blue light conditions. Double mutant analyses demonstrated that pif1phyA, pif1phyB, pif1cry1 and pif1cry2 have enhanced cotyledon opening compared to the single photoreceptor mutants under diurnal blue light conditions. Taken together, these data suggest that PIF1 functions as a negative regulator of photomorphogenesis and that light-activated phys induce the degradation of PIF1 through the ubi/26S proteasomal pathway to promote photomorphogenesis. / text