The heteroduplex mobility assay (NMA) reveals sequence dissimilarity between DNA by measuring the retarded migration of the hybrid or heteroduplex using polyacrylamide gel electrophoresis, Heterogeneity in some cultures of toxic dinoflagallates of the genus Alexandrium (Halim) Balech was observed during comparison of the amplified D1-D2 region of the large subunit rRNA gene (rDNA) using this method. HMA also allowed grouping of clones obtained from toxic bloom events in the Chilean, southernmost Pacific within the Asian Southern Pacific lineage of A. catenella (Whedon et Kofoid) Balech. The applied methodology provides a rapid and simple tool for use in assessing heterogeneity as well as for molecular grouping of strains among the genus Alexandrium.010FONDEFpuribe@machi.med.uchile.cl04FONDEF35