Isorhizochalin (1) was isolated as its peracetate from the EtOH extract of the sponge Rhizochalina incrustata. Its structure and absolute stereochemistry were elucidated as (2S,3R,26R,27R)-2,27-diamino-3-O-beta-D: -galactopyranosyl-oxy-26-hydroxyoctacosan-18-one by extensive NMR, MS studies, chemical transformations, including micromolar-scale Baeyer-Villiger oxidation, and by analysis of CD spectra of isorhizochalinin perbenzoate (2b). Isorhizochalin is an unprecedented C-2 epimer of rhizochalin having an erythro configuration at the glycosylated 2-amino-3-alkanol alpha-terminus in contrast with a canonical threo configuration for other representatives of this structural group. Probable biogenesis of 1 is discussed in the context of known sphingolipid biosynthesis beginning with condensation of alanine with a fatty acyl CoA thioester. The aglycone, isorhizochalinin (2a), shows cytotoxicity against human leukemia HL-60 and THP-1 cells with IC(50) values of 2.90 and 2.20 microM, respectively.