The endogenous respiration of six members of Saprolegniaceae (Oomycetes), Saprotegnta sp., Thrau.stotheca sp., Achlya sp., Dlchtynchiis sp., Aphanomgces eiiteiehes, and .4. astaci were studied in the presence and in the absence of exogenous substrate using conventional manometric techniques. Glucose stimulated the rate of nxygen uptake of unstarved mycelia to some extent in all the fungi. Attempts to increase tbe weak stimulation of respiration by glucose in ^1. a.staci were not successful. The respiratory quotients of tbe fungi tested were usually in the range of 0.7 to 0.9 (luring endogenous respiration, and addition of glucose iDCreased tbese values inure tban expected.L-leucine and L-glutamic acid stimulated respiration of .4. astaci only when tbe liingus was starved, and acetic acid and butyric acid were inhibitory. Fructose and :icetic acid increased respiration in starved mycelium of .4. eutciches while L-leucine and i.-glutamic acid had little effect. Antimycin A, IIOQNO, HCN, and fluoroacetate strongly inbibited endogenous oxygen uptake by .4. astaci. Amytal and azide were also markedly inbibitory while rotenone and CO had little effect. DNP and diphenylamine inbibited respiration at a bigb concentration but at a lower concentration DNP was stimulatory. In contrast the respiration of Saproleynia s[). was resistant to cyanide, antimycin A, and HOQNO.Spectrophotometric observations on homogenized mycelia of Saprolegnia sp. and of .4. astdci indicated the presence of cytochrome c (55! nni}, two 'j-type cytochroines (557 and 564 nm) and cytochrome a-a^ (605 nm) all in apj)roximately equimolar concentrations. In both strains CO combined with cytochrome a^ and an unidentified pigment. A remarkable similarity in tbe cytochrome system seems to exist between these two strains and some members of the Leptomitales. 38 Phvsiol. Plant.. 21. 1968 [573] Pkysiol. Plant.. 21.196S