ECOLOGY. For the article “Diverse taxa of cyanobacteria produce β-N-methylamino-l-alanine, a neurotoxic amino acid,” by Paul Alan Cox, Sandra Anne Banack, Susan J. Murch, Ulla Rasmussen, Georgia Tien, Robert Richard Bidigare, James S. Metcalf, Louise F. Morrison, Geoffrey A. Codd, and Birgitta Bergman, which appeared in issue 14, April 5, 2005, of Proc. Natl. Acad. Sci. USA (102, 5074–5078; first published April 4, 2005; 10.1073/pnas.0501526102), the authors note that on page 5074, the first sentence of the second full paragraph, right column, should read “BMAA was recently discovered in the brain tissues of eight of nine Alzheimer's patients,” and not “BMAA was recently discovered in the brain tissues of nine Canadian Alzheimer's patients.” The authors also note on page 5075, first line in the right column, “Lyophilized samples (5–20 μg dry weight)” should read “Lyophilized samples (5–20 mg dry weight).” In addition, on page 5076, the unit quantification “mol” in both columns (line 8 in left column; line 1 in right column) should read “μmol.” These errors do not change the conclusion of the article.