A large model trapezoidal steel box girder containing three stiffened diaphragms was tested in the Laboratory as part of the 'Merrison' program of research. The diaphragms contained differing amounts of transverse stiffening. Each diaphragm region was tested separately and strains, deflections and modes of collapse recorded. The details of fabrication, initial measurements and test procedures are described and the behaviour of the diaphragm regions discussed. The elastic stresses are compared with the results of finite element analyses and the collapse loads with panel and stiffener strengths calculated using Part 3 of the 'Merrison' Interim design and Workmanship rules.