Raman spectra of mitochondria in living budding yeast cells (zygote of Saccharonlyces cerevisiae and Saccharomyces bayanus) have been recorded with 532 nm excitation. Strong and sharp Raman bands are observed at 1584, 1315, 1129, 749, and 601 cm(-1), in addition to known phospholipid bands. These Raman bands are not observed in the corresponding Raman spectrum of mitochondria obtained with 632.8 nm excitation. Their peak positions and relative intensities agree excellently with the reported resonance-enhanced Raman bands of ferrous cylochrome c, which has a Q-band absorption at 520 nm. We have thus succeeded in detecting ferrous cytochrome c, one of the key intermediates in the electron-transport chain in mitochondria, in vivo without any pretreatment. In vivo resonance Raman detection of ferrous cytochrome c opens up new possibilities for real-time and quantitative tracing of respiration dynamics in mitochondria in a single living cell.