We have analyzed Leishmania tarentolae mitochondrial ribonucleoprotein (RNP) complexes using the 9S small subunit (SSU) rRNA and the 12S large subunit (LSU) rRNA as markers, and have identified a 50S RNP particle as the putative mitochondrial monosome, a 40S particle as the putative LSU and a 30S particle as the putative SSU. These assignments are supported by morphological analysis by cryo-electron microscopy and proteomics analyses by mass spectrometry. The presence of additional rRNA-containing particles complicated the analysis and most likely was the basis for previous difficulties in identification of these ribosomes; thus, in addition to the monosomes and their subunits, there are abundant stable 45S particles (SSU(*)) containing only 9S rRNA, which may represent homodimers of the SSU or SSU associated with additional proteins, and variable minor amounts of 65S and 70S particles, which represent homodimers of the LSU and SSU(*), respectively. These additional rRNA particles might be due to the lengthy mitochondrial isolation and ribosome isolation procedures or may be present in vivo and play yet undetermined roles.