An experimental aeration system, called a pump-stirred aerator (PSA) was tested. The PSA consisted of an 11 kw centrifugal pump circulating liquid to and from a 10 msuperscript 3 tank and injecting air at the pump inlet. Air was admitted directly into the centrifugal pump as it was pumping liquid pig slurry. The mass transfer coeff. of oxygen into water (KLa) was measured and then pig slurry used to obtain similar results. The max. gas-liquid mass transfer coeff. (KLa) achieved for the water, pig slurry and separated pig slurry were .0013, .0043 and ..0046 sec-1 resp. The oxygen transfer efficiencies were 0.13, 0.22 and 0.37 kg/kwh for the water, unseparated and separated pig slurry resp. These values are comparable to efficiencies achieved with the venturi-type aerator.