The 16s rDNA sequences of 20 novel isolates of members of the order Planctomycetales were compared to those of the type strains of described planctomycete species and 22 planctomycete isolates for which the 16s rDNA sequences had been previously determined. The novel isolates could be assigned to several phylogenetically broad groups, four of which are defined by the genera Gemmata, lsosphaera, Planctomyces and Pirellula. To evaluate polyamines as a chemotaxonomic marker within this order, the polyamine pool was determined for six planctomycete reference species and for 20 planctomycete isolates. All analysed members of the order Planctomycetales contained significant amounts of polyamines. sym-Homospermidine (HSPD) is present in all strains except Planctomyces limnophilus and related strains, which had high amounts of putrescine (PUT) as the dominant polyamine component. The distribution of PUT, HSPD and spermidine reflects the phylogenetic diversity within the Planctomycetales as closely related representatives of the phylogenetic groups defined by described species and novel isolates exhibit similar polyamine patterns. Determination of the DNA base composition revealed G+C contents of >60 m o l % for members of Gemmafa and lsosphaera whereas, except for two isolates, strains which are phylogenetically associated with Planctomyces and Pirellula had G+C contents of 51-57 mol%.~~