Electron microscopy of native and dimethyl-suberimidate crosslinked rabbit muscle phosphofructokinase (EC 2.7.1.11; ATP:D-fructose-6-phosphate 1-phosphotransferase) has been carried out using negative staining with sodium phosphotungstate. The results obtained suggest the protomer of molecular weight 80,000 can be approximated as a prolate ellipsoid with axes of 67 A and 25 A. The dimer, which is the fundamental unit for polymerization, is formed by association along the 25 A axis and has approximately dimensions of 67 A X 55 A X 25 A. The tetramer appears to be formed by an end-to-end aggregation of dimers, and the octamer is a sheet-like structure made up of a side-to-side aggregation of tetramers. Higher crosslinked aggregates and long crosslinked filaments also are seen. The filaments have a constant width of about 250 A, are about 0.5 mum in length, and appear to involve tetramers as a fundamental structural unit. The functional significance of the filament structure is not known.