Microbial production of 5-aminolevulinic acid (ALA) by photosynthetic bacteria compared to other bacteria and algae is reviewed. During aerobic-microaerobic cultivation of Rhodobacter sphaeroides mutant strain CR520, control of the redox potential was effective for producing large amounts of extracellular ALA. ALA has been practically applied in agriculture as an herbicide, an insecticide and a growth-promoting factor for plants. New agricultural applications including salt tolerance and cold temperature tolerance of plants are also described. Finally, recent medical applications for cancer treatment, tumor diagnosis and other clinical uses are discussed.