We report that Corynebacterium glutamicum colonies exhibit a developmental transition in culture. When cultured on a routinely used complete medium (CM2B), this bacterium first formed a flat translucent colony. Subsequently, some parts of this colony expanded to form small spherical yellow colonies that finally developed into a single large yellow colony. The small flat colony consisted of long thick cells, which were occasionally V or Y shaped, while the large yellow colony consisted of short small rods. A similar colony development pattern was observed in Corynebacterium ammoniagenes and Corynebacterium callunae. Analysis following shotgun cloning revealed that the introduction of a multi-copy-number plasmid carrying amtR, a global transcriptional regulator for nitrogen metabolism, into C. glutamicum cells induced precocious colony development. An amtR-null C. glutamicum mutant exhibited delayed development. Detailed observations of C. glutamicum cells cultured on CM2B medium containing buffers at various pH values revealed that the colony growth was rapid at a pH value of 6.4 or higher and slow but distinct at a pH of less than 6.4. This pH threshold increased to 6.8 following the addition of 0.1% glucose into the medium.