We have produced defined fragments of the oat PhyA AP3 protein using an in vitro translation system and analyzed the quaternary structure of these fragments by size exclusion chromatography. Sequences between amino acids S599 and L683 are shown to dimerize by this in vitro assay and by a repressor-based in vivo assay. A subset of this dimerization region, V623 to S673, which has previously been identified as being involved in interdomain interactions based on the behavior of overlapping constructs in a repressor assay for protein-protein interaction is shown by both assays to be necessary but insufficient for dimerization. Sequences between L685 and R815, which are unable to dimerize by themselves, are shown to interact with sequences between S599 and L683. Sequences E1069-Q1129, also previously suggested to be involved in dimerization, are shown here not to be required for phytochrome dimerization. These results based on an in vitro assay have confirmed some of the results previously obtained using an in vivo assay and extend these results by revealing new protein-protein interactions. This dissection of sequences involved in phytochrome dimerization taken together with previous work has enabled us to propose a model for the behavior of the dimerization region where core structure involved in dimerization is located on both sides of a region around residue 750 found at the surface.