Bemisia tabaci B biotype whiteflies are harmful to a variety of crop species because, as vectors of plant viruses, they ingest virions as they feed on the phloem of infected plants, and transmit them when they feed on uninfected plants. As part of our ongoing interest in the digestive systems of plant feeding insects, we have demonstrated significant activity of cysteine proteases at the protein level. However, our most recent findings suggest that there is a mixture of more than one family of proteases. Because we know that the proteolytic pH optimum in whole whitefly extracts is at pH 5.5, we feel that the proteases must be able to function in acidic environments. Using Cathepsin B, an acidic cysteine protease commonly found in Hemipterans, as a positive control, we have optimized our protease inhibition assays. Based on the knowledge of a cDNA for the same enzyme from a related Homopteran rice pest, Nilaparvata lugens, and related insect sequences, we have also designed PCR primers that have allowed us to ‘fish’ in genomic DNA preparations. Future experiments will focus on developing probes that will eventually lead to a characterization of the gene.