A study was made of the quantitative composition of the microbial communities living on detrital particles derived from the turtle grass Thalassia testudinum. The number of organisms on and the rate of oxygen consumption of the detritus are approximately proportional to the total surface area. Field samples of detritus harbored about 3 × 109 bacteria, 5 × 107 flagellates, 5 × 104 ciliates, and 2 × 107 diatoms and consumed from 0.7 to 1.4 mg 02/hr per g dry wt.The detritus‐consuming amphipod Parhyalella whelpleyi feeds on detrital particles and on its own fecal pellets but it only uses the microorganisms; the dead plant residue passes undigested through the intestine. After a few days the microbial communities living on fecal pellets are qualitatively and quantitatively comparable to those living on other detrital particles.The amphipods decrease the particle size of the detritus thereby increasing its total surface and thus the microbial activity. In less than 4 days, the mechanical activity of the amphipods may increase the detrital O2 uptake by 110% of their own metabolic rate. Measurements of respiratory rates of the amphipods will therefore give a much too low estimate of their total role in the ecosystem.