A new α,α-disubstituted α-amino acid derivative, dysibetaine (1), was isolated from an aqueous extract of the marine sponge Dysidea herbacea collected in Yap, Micronesia. The structure of 1, (2R*,4R*)-2- (trimethylammonium)methyl-4-hydroxy-5-oxoprolinate, was determined by spectral methods and X-ray crystallography. © 1999 Elsevier Science Ltd. All rights reserved.