The crystal structure of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) fromArabidopsis thalianais reported at 1.5 Å resolution. In light of the importance ofA. thalianaas a model organism for understanding higher plant biology, and the pivotal role of Rubisco in photosynthetic carbon assimilation, there has been a notable absence of anA. thalianaRubisco crystal structure.A. thalianaRubisco is an L8S8hexadecamer comprising eight plastome-encoded catalytic large (L) subunits and eight nuclear-encoded small (S) subunits.A. thalianaproduces four distinct small-subunit isoforms (RbcS1A, RbcS1B, RbcS2B and RbcS3B), and this crystal structure provides a snapshot ofA. thalianaRubisco containing the low-abundance RbcS3B small-subunit isoform. Crystals were obtained in the presence of the transition-state analogue 2-carboxy-D-arabinitol-1,5-bisphosphate.A. thalianaRubisco shares the overall fold characteristic of higher plant Rubiscos, but exhibits an interesting disparity between sequence and structural relatedness to other Rubisco isoforms. These results provide the structural framework to understandA. thalianaRubisco and the potential catalytic differences that could be conferred by alternativeA. thalianaRubisco small-subunit isoforms.