Lysozymes are enzymes characterized by their ability to break down bacterial cell walls. In insects certain lysozymes are only found in the midgut, whereas others are only found in the haemolymph and fat body after immune challenge. We identified two lysozyme-encoding cDNAs from Aedes aegypti. Both deduced protein sequences are basic in nature, contain 148 amino acids including eight highly conserved cysteine residues, and their genomic sequences contain a single intron. Transcriptional profiles indicated that the predominant form is constitutively expressed and up-regulated upon immune challenge and blood feeding in adult mosquitoes. The second form is expressed during early developmental stages, larvae and pupae, and at low levels in adults after immune challenge. Lysozymes in Aedes aegypti play both roles, defined by the spatial and temporal regulation of their expression.