There are at least six small stable RNAs in Mycoplasma capricolum cells besides tRNAs and rRNAs. One of them, MCS5 RNA, is a homolog of RNase P RNA. The predicted secondary structure of this RNA is essentially the same as that of other eubacterial RNase P RNAs. MCS5 RNA is more similar to the RNase P RNA of B. Subtilis than to that of E. coli. This is consistent with previous conclusions that mycoplasmas are phylogenetically related to the low G + C Gram-positive bacterial group. The major substrates for MCS5 RNA must be the precursors of tRNAs. The precursor of MCS6 RNA, which is a homolog of the E. coli 10Sa RNA, may also be a substrate for the MCS5 RNA because this RNA has a tRNA-like structure at its 5' and 3' ends.