The aspartic protease cathepsin D (Clan AA, Family A1) is expressed in the schistosome gut where it plays an apical role in the digestion of hemoglobin released from ingested erythrocytes. In this report, RNA interference approaches were employed to investigate the effects of knockdown of schistosome cathepsin D. Cultured schistosomules of Schistosoma mansoni were exposed by square wave electroporation to double stranded RNA (dsRNA) specific for cDNA encoding S. mansoni cathepsin D. RNAi-mediated reductions in transcript levels led to phenotypic changes including significant growth retardation in vitro and suppression of aspartic protease enzyme activity. In addition, black-pigmented heme, the end point by-product of normal hemoglobin proteolysis that accumulates in the schistosome gut, was not apparent within the guts of the treated schistosomules. Their guts appeared to be red in color, rather than black, apparently indicating the presence of intact rather than digested host hemoglobin. These phenotypic effects were apparent when either of two forms of dsRNA, a long form spanning the entire target transcript or a short form specific for the 3'-region was employed. Off-target effects were not apparent in transcript levels of the gut-localized cysteine protease cathepsin B1. Finally, cathepsin D may be an essential enzyme in the mammal-parasitic stages of schistosomes because schistosomules treated with dsRNA did not survive to maturity after transfer into Balb/c mice. These and earlier findings suggest that, given its essential function in parasite nutrition, schistosome cathepsin D could be developed as a target for novel anti-schistosomal interventions.