An alkaloid extracted from a small evergreen tree, Annona montata, was identified as 1-(2-amino-4-pyrim-idinyl)-β-carboline monohydrate by the X-ray method. The crystal data are; a=20.699, b=4.453, c=15.720A, and β=112.160; space group, P21/c; Z=4. The structure was solved by the direct method and refined by the least-squares method to the final R value of 0.109 for 1451 independent reflections. The water molecule is connected to four nitrogen atoms by hydrogen bonds. Two of them are in different groups; one is in the five-membered ring of the β-carboline skelton, and the other is in the pyridine of the β-carboline skelton. The fifth hydrogen bond connects two nitrogen atoms, one of which is in the amino group, and the other, in the py-rimidine skelton.