Abstract The oogonia and oospores of 80 isolates, representing 40 species of Pythium, were examined to determine the taxonomic values of direct measurements and derived indices. It was demonstrated that 20 individual oogonia from an isolate provided a suitable data base, with acceptable levels for standard errors of the means. All four measurable and all three calculable variables showed continuous variation between isolates and species, so that no single criterion was able to effect taxonomic separation. The inadequacy of the traditional plerotic/aplerotic concept was revealed. Derived indices were shown to have a significant taxonomic value in addition to the measured parameters from which they were derived. When taken together, using canonical variate analysis, the variables enabled isolates to be grouped into their respective species without recourse to other taxonomic features. The enhanced precision of the assessment of oogonial characteristics has enabled a new approach to Pythium classification to be developed.