The non-protein amino acid, β-N-methylamino-L-alanine (BMAA), has been reported as an important pathogenic factor in the neurodegenerative disease amyotrophic lateral sclerosis-parkinsonism dementia complex (ALS/PDC).BMAA has been reported in various species of cyanobacteria living in freshwater and marine environments.An isomer of BMAA, 2,4-diaminobutyric acid (DAB), has also been found in a variety of cyanobacteria.In the present study, two strains of Microcystis aeruginosa and one strain ofNostoc sp.were cultured using a BG-11 medium recipe altered with different levels of nitrate and phosphate.The influence of temperature and illumination intensity was also examined.An additional 17 strains of cyanobacteria isolated from fresh waters in China, including Nostoc sp., Calothrix sp., Scytonema sp., Lyngbya sp., Synechococcus sp., Anabaena sp., Planktothrix sp., Aphanizomenon sp., etc., were incubated under normal conditions with BG-11 medium.Three field samples were also collected from lakes in China.Liquid chromatography-mass spectrometry (LC-MS/MS) was used to directly determine BMAA and DAB levels within cyanobacterial samples.No BMAA was detected in any cyanobacteria strains, under any of the incubation conditions tested.However, trace levels of DAB were found in most samples.The results indicated that production of DAB was affected by the environmental factors studied and by the ratio of nitrate and phosphate.The occurrence of DAB in freshwater environments, and its significance needs to be considered in terms of protecting human health and drinking water supplies.