Increased consumer demand for yogurt and Italian cheeses, particularly Mozzarella, over the past decade has intensified production demands on thermophilic starter cultures. Dramatically elevated production rates within existing facilities have created problems analogous to those experienced years ago by the dairy industry for mesophilic lactococci when Cheddar cheese production increased dramatically. In contrast to mesophilic lactococci, however, diversity among lactic genera that contain thermophilic species has impeded the progress of microbiological research required to meet these challenges. For example, the bacteriophages that attack these cultures are becoming an important commercial problem, yet little is known about them. Another unique characteristic of thermophilic lactic cultures is the microbial interaction between lactobacilli and streptococci cultured together. Further characterization of this interaction, particularly its effects on physical characteristics of the product, would benefit the cheese industry. Diversity of proteolytic and peptidolytic enzymes exists within lactobacilli. These enzymes are important in cheese flavor and texture development, therefore, further characterization of these systems would facilitate manipulation of organoleptic and physical properties of products. Finally, utilization of other, less studied thermophilic species would facilitate manufacture of new products and improved quality or manufacturing processes.