Results of competition experiments with one growth-limiting factor under idealized experimental conditions have been reported extensively, and usually provide ample support for the conclusion that 'complete competitors cannot coexist'. However, under conditions of multiple substrate limitation and discontinuous or alternating supply of nutrients, coexistence of species is quite common. Since such patterns of nutrient supply may be expected to prevail in many natural environments the mechanisms ruling the survival and growth of bacteria under such conditions need to be understood. However, it appears that surprisingly little is known of the physiological state of individual competing species grown in mixed cultures. Unfortunately, basic information such as the actual concentration of limiting nutrients is lacking in most cases. But perhaps the recent development of new and powerful techniques to explore the physiological properties even of individual cells will further stimulate studies into the mechanisms behind the competitiveness of microbial species.