Penetration of a motile microbe, Enterobacter aerogenes, was investigated through aseptic packages filled with milk, meat soup, sausage gravy and soft agar. Penetration was dependent on the microhole diameter and on the viscosity of the foodstuff in the package. The smallest microhole diameter permitting the passage of E. aerogenes in this study was 5 μm when the packages were filled with milk and the microhole length was 40 μm. While the packages were filled with soft agar, meat soup and sausage gravy, the smallest penetrable microhole was 10, 20 and 25 μm in diameter, respectively. On the other hand, even the microholes of 100μm diameter were not penetrated by E. Aerogenes in every package tested. This may have been owing to blockage of the microholes by the test foodstuff thus hindering the penetration of bacteria.