Many Pseudo-nitzschia species produce the toxin domoic acid, which accumulates in the food web during blooms, sometimes causing amnesic shellfish poisoning (ASP) in higher trophic levels, including humans. In addition, Pseudo-nitzschia species have been reported to form long-lasting monospecific blooms, and a possible explanation for this could be allelopathic effects of the toxin, since domoic acid has been detected in high amounts in the surrounding medium in stationary growth phase. We therefore examined the potential allelopathic effects of P. multiseries and its toxin domoic acid. In mixed-batch culture studies of domoic acid-producing P. multiseries and the algal test species Chrysochromulina ericina, Heterocapsa triquetra, Eutreptiella gymnastica and Rhodomonas marina, no allelopathic effects of P. multiseries were found. Different growth results using 2 different strains of P. multiseries grown with C, ericina were explained by minor differences in pH. Bioassays testing the effect of domoic acid itself on 9 different phytoplankton species, namely C. ericina, E. gymnastica, Karenia mikimotoi, H. triquetra, Heterosigma akashiwo, Prorocentrum minimum, P. micans, Pyramimonas propulsa and R. marina confirmed a lack of allelopathic effects of the toxin. This lack of allelopathic effect of the shellfish-poisoning toxin domoic acid seems to correspond with the results of phytoplankton species causing DSP (diarrheic shellfish poisoning) and PSP (paralytic shellfish poisoning), where it appears that these shellfish-poisoning toxins do not cause allelopathic effects either.