SUMMARYNigeran, a hot-water-extractable glucan found in the hyphal walls of certainAspergillusandPenicilliumspecies has been utilized as a biochemical marker to study their phylogenetic relationships. The presence or absence of nigeran in members of certain previously assigned taxonomic groups suggests that some of these taxa may not be natural. The amount of nigeran deposited in the walls of competent species can vary over a 100-fold range under the experimental conditions employed. Of all species examined which contain nigeran, onlyPenicillium meliniiis capable of degrading its endogenous polysaccharide.