Abstract—We confirmed that after extraction in the absence of added Mg2+, a small fraction of phytochrome was associated with pelletable, hydrophobic membranes. When microsomal material of several plant species was subjected to Triton X‐114 phase partitioning, a part of phytochrome migrated into the hydrophobic Triton phase in contrast to soluble phytochrome. The amount of bound phytochrome partitioning into the Triton phase varied from 6% for oats to 30% for zucchini and 50% for mustard and maize (0.5–10% of total phytochrome). Membrane‐associated phytochrome could be solubilized by the zwitterionic detergent CHAPS and with the nonionic detergent dodecylmaltoside. Subjected to gel filtration on Superose‐6/FPLC, oat phytochrome of the CHAPS solubilized sample was eluted in three different molecular weight ranges. There was a main fraction with the molecular weight of purified phytochrome, another fraction (approximately 20%) with a higher molecular weight, and a third small fraction appearing immediately after the void volume. Gel filtration after solubilization by dodecylmaltoside resulted in two distinct fractions: the one eluted at the position of the phytochrome dimer, and the other (approximately 15%) with an apparent molecular weight of 800 kDa. Phytochrome was detected, separated and quantified by SDS‐PAGE, and western blotting with the monoclonal antibody Z‐3B1. We assume that the distinct, phytochrome positive, high molecular weight fractions contain phytochrome associated with hydrophobic protein.