The maximum cell yield (YMc - g biomass carbon per gram substrate carbon) and the rate of maintenance metabolism (mc - g substrate carbon/g biomass carbon per hour) have been determined for substrate limited continuous cultures of Pseudomonas fluorescens and P. aeruginosa. The metabolism of the organic substrates was monitored by measuring the COD-removal1) rates at different dilution rates. The advantages of expressing yield and maintenance coefficients in carbon units is discussed.