1. Mitochondria isolated from Polytomella caeca contain cytochromes b, c+c(1) and a+a(3) and several flavoprotein species. 2. Electron transport is inhibited by antimycin A, rotenone, piericidin A and cyanide. 3. Spectral data indicate that antimycin A inhibits the reoxidation of reduced cytochrome b. 4. Various types of flavoprotein are characterized by simultaneous spectrophotometric and fluorimetric measurements on antimycin A-inhibited preparations and also by their absorption and fluorescence-emission spectra. 5. The rotenone-sensitive site lies between the two flavoproteins of the respiratory chain, designated FpD1 and FpD2. 6. Other flavoprotein species detected include those involved in the oxidation of succinate and externally added NADH; a large proportion of mitochondrial flavine is reduced by dithionite but not by known respiratory substrates. 7. The kinetics of flavoprotein and cytochrome reactions were studied.