Microalgae have previously been reported to contain β-N-methylamino-l-alanine (BMAA), and the global presence of these primary producers has been associated with the widespread occurrence of BMAA in marine organisms. It has been repeatedly shown that filter-feeding bivalves accumulate phytoplankton species and their toxins. In this study, the concentrations of total soluble BMAA and DAB as a function of growth phase were observed for four non-axenic diatom species (i.e. Phaeodactylum tricornutum, Chaetoceros sp., Chaetoceros calcitrans and Thalassiosira pseudonana). These strains had previously been shown to contain BMAA using a highly selective HILIC-MS/MS method. BMAA cell quota appeared to be species-specific, however, highest BMAA concentrations were always obtained during the stationary growth phase, for all four species, suggesting that BMAA is a secondary metabolite. While DAB was detected in a bacterial culture isolated from a culture of P. tricornutum, the presence or absence of a bacterial population did not influence production of BMAA and DAB by P. tricornutum, i.e. no significant difference was noted for BMAA and DAB production between axenic and non-axenic cultures. The presence of DAB in bacteria had previously been shown, and raised the question as to whether DAB observed in many species of microalgae may arise from the non-axenic culture conditions or from the microalgae themselves.