Ribosomal RNA genes were isolated from the germinated conidial and mycelial cells of N. crassa by repeated cycles of 3H-DNA:rRNA reactions followed by hydroxyapatite chromatography. Specificity of multiple copies of those rDNAs with respect to N. crassa cell types was studied. The fraction of N. crassa germinated conidial in vitro labelled 3H-DNA recovered in the presence of rRNA isolated from the same cell type was about 2.2%, when compared with approximately 1.2% rDNAs obtained in mycelial cells. These isolated rDNAs reacted specifically to 26S and 17S rRNAs of eukaryotic (N. crassa) organisms and did not react with 4S tRNAs. rRNA:rDNA reassociation kinetics studies indicate that 90% of the rRNA genes were homogeneous and not identical with the other 10% rRNA genes isolated from N. crassa mycelia. These studies suggest that the possible heterogeneity of rDNA sequences of N. crassa cannot be attributed to inclusion of any tDNA sequences as has been shown in the heterogeneity of rDNA sequences of the bacterium Escherichia coli. The heterogeneity of multiple copies of N. crassa rDNAs could be due to differences in internal or external spacer regions of N. crassa rRNA genes.