The interest of this research concerns the defense barriers of crustaceans to disease. The region under investigation in the prawn, Sicyonia ingentis , was the midgut trunk (MGT) and its product of the peritrophic membrane (PM). This region is of particular interest because it is not lined by an exoskeleton like the rest of the body. The PM forms a wrapper around food 1 ?m or larger to be discharged by the body. The various chemical, histological and microscopy utilized were for the purposes of determining the composition and permeability of this layer. The chitosan reaction, base/acid exposures, enzyme incubations and histological stains helped to determine that the PM consists of carbohydrates, namely chitin. The permeability of the PM was tested with exposure of PMs to various sized tracers. Although the permeability to viruses is still to be determined, SEM and LM strongly support that bacteria cannot pass through an intact PM.