AbstractWe have isolated cDNA clones encoding the full‐length Drosophila melanogaster cysteine proteinase 1 (CP1). The clones were isolated from the Drosophila melanogaster haemocytic mbn‐2 cell line, where the gene is relatively strongly expressed, giving a transcript of 1.6 kb in size. We present the sequence encoding the full‐length protein, and deduce the genomic organlzation of the gene by comparison to previously published genomic partial sequence data. Immunofluorescence shows that CP1 is localized in small granules, probably lysosomes, in mbn‐2 cells. The data presented suggest a role for cysteine proteinases in immune functions in insects. It is likely to participate in the degradation of internalized material in phagocytic cells.