Four cereal grains (whole wheat, corn flour, rice flour and milo) were examined before and after being subjected to two extrusion processes to determine the effects of extrusion processing on in vivo energy digestibility. Utilizing chromic oxide as an inert marker, apparent digestibility coefficients of the cereal grains were evaluated with 13 g Penaeus vannamei. The cereal grains were extruded with a Wenger Extruder (model TX52) under two conditions allowing the production of wet and dry pellets. Test diets for each test ingredient were prepared containing 70% semi-purified reference diet and 30% of the test ingredient. Extrusion processing resulted in increased gelatinization of each of the tested cereal grains with wet extrusion producing the highest values. Processing and/or gelatinization values did not always correspond to increased digestibility of the test ingredients. Highest apparent digestible energy (ADE) values for wheat occurred without extrusion and under dry conditions. Highest ADE values for rice flour occurred without extrusion and under wet extrusion conditions. Dry extrusion of the corn flour and milo resulted in the best energy digestibility coefficients for these grains. However, wet extrusion appeared to significantly reduce the availability of energy from corn flour. Based on these results, extrusion did result in increased ADE values of poorly utilized cereal grains; however, no one extrusion condition was optimal for all cereal grains tested.