SUMMARY In Drosophila flight muscles, glycolytic enzymes are co-localized along sarcomeres at M-lines and Z-discs and co-localization is required for normal flight. We have extended our analysis of this phenomenon to include a set of six glycolytic enzymes that catalyze consecutive reactions along the glycolytic pathway: aldolase, glycerol-3-phosphate dehydrogenase (GPDH),glyceraldehyde-3-phosphate dehydrogenase (GAPDH), triose phosphate isomerase,phosphoglycerate kinase and phosphoglycerol mutase (PGLYM). Each of these enzymes has an identical pattern of localization. In mutants null for GPDH,localization of none of the other enzymes occurs and therefore is interdependent. In optimally fixed preparations of myofibrils, accumulation of the enzymes at M-lines is much greater than at Z-discs. However, localization at M-lines is more labile, as shown by loss of localization when fixation is delayed. We have begun to analyze the protein—protein interaction involved in glycolytic enzyme co-localization using the yeast two-hybrid system. We have identified two pair-wise interactions. One is between GPDH and GAPDH and another is between GPDH and PGLYM.