In continuation of research on two-headed sphingolipids from the marine sponge Rhizochalina incrustata, a new aglycon named rhizochalinin A (1) was isolated from the acid hydrolysis product of total natural galactosylated sphingolipids of the sponge (collected in Madagascar in 1986) via solvent extraction and column chromatography (Polychrom, silica-gel, Sephadex LH-20), yielding 27.9 mg (0.05% of the sponge's dry weight). The structure of 1 (molecular formula C₃₁H₆₂N₂O₅) was determined using spectral techniques (PMR, ¹³C NMR, DEPT, COSY, HSQC, HMBC, MALDI-TOF mass spectrometry) and chemical transformations (e.g., Baeyer—Villiger oxidation of its triacetate derivative), revealing it to be a pseudodimeric amino alcohol with a carbamoyl group bonded to C-27 and a carbonyl group at C-18. Rhizochalinin A showed antileukemic activity against human leukemia THP-1 cells with an IC₅₀ value of 7.5 µM.