We describe the synthesis of 10-aza-9-oxakalkitoxin, an N,N,O-trisubstituted hydroxylamine-based analog, or hydroxalog, of the cytotoxic marine natural product kalkitoxin in which the -NMe-O- moiety replaces a -CHMe-CH2- unit in the backbone of the natural product. 10-Aza-9-oxakalkitoxin displays potent and selective cytotoxicity (IC50 2.4 ng mL(-1)) comparable to that of kalkitoxin itself (IC50 3.2 ng mL(-1)) against the human hepato-carcinoma cell line HepG2 over both the human leukemia cell line CEM and the normal hematopoietic CFU-GM. Like kalkitoxin, and contrary to the common expectation for hydroxylamines, 10-aza-9-oxakalkitoxin is not mutagenic.