A biological sterilization indicator (BI) is provided. The BI can include a housing, and a container positioned in the housing. The BI can further include a first chamber, and a second chamber with a source of biological activity disposed therein. The BI can further include a substrate positioned in the housing between the first chamber and the second chamber. The substrate can be positioned in fluid communication with the first chamber and the second chamber, and the substrate can be further positioned such that the substrate is not in direct contact with the source of biological activity. The substrate can comprise a hydrophobic nonporous film having an aperture. The substrate and the aperture are dimensioned to control fluid flow between the first compartment and the second compartment. The substrate can extend around substantially the entire inner perimeter of the housing between the first compartment and the second compartment.