ABSTRACTThe structural organization of DNA in the plastids of two anomalously pigmented dinoflagellates,Glenodinium foliaceumStein andGyrodinium aureolumHulburt, was determined using the DNA‐specific fluorochrome DAPI and correlated with TEM observations. The plastids ofG. foliaceumwere found to possess both a peripheral DNA ring and isolated point nucleoids. This arrangement was shown to be similar to that of the diatomAsterionella formosaHass. and may be characteristic of the Bacillariophyceae.G. aureolumexhibited a novel distribution of plastid. DNA as one or two beaded bands, whereas the plastids of the similarly pigmented haptophyte,Emiliania huxleyi(Lohm.) Hay&Mohler, possessed scattered point nucleoids. These findings support the idea thatG. foliaceumharbours an endosymbiotic diatom, but suggest that the plastids ofG. aureolumandE. huxleyiare unrelated. The use of plastid DNA configuration as a phylogenetic marker is considere