As a part of studies on physiologically actfve substances produced by mushrooms, we examined constituents of.Clitocybe acromelalga (Japanese name Dokusasako). It has long been known in Japan that accidental ingestion of this toadstool produces markedly increased hyperemfa and hyperesthesia in fingers and toes. We describe here isolation and characterization of a new pyridine nucleoside 1 for which we suggest the name clitidine. Crude clitidine was obtained in 0.036% yield from frozen fruit bodies through sequential extraction (H20), precipitation (acetone), dialysis and chromatography on Sephadex G-10. The crude crystals were further purified by preparative paper chromatography (n-BuOH-H20 saturated), paper electrophoresis at pH 4.6 (Py-AcOH buffer) and finally by recrystallization from water. The pure sample had elemental composition CllH1406N2.H20, mp 189-191", [~$~~-50.6" (H20, c= 10.6mg/ml), exhibited uv maxima at 271nm (loge 4.09, H20), 267nm (4.16, pH 21, 271nm (4.18, pH 121, and showed ir absorption bands at 3300-2200, 1660, 1585, 1065, 1030 cm" (nujol). The nmr spectrum (in IMSO) indicated presence of a pentose moiety [S 3.70 (2H, bs). 4.08 (3H, bs), 5.59 (lH, d, J=5Hz)] and a 3,4-disubstituted pyridine ring [S 6.90 (lH, d, J=7Hz), 8.24 (lH, dd, J=7, 1.51, 8.7 (lH, d, J=l.5)3. The S values of aromatic protons suggested that the substituents at C-3 and C-4 are electron attractive and electron donating respectively. Therefore, structure1 orLwas tentatively assigned to clitidine. Of these alternatives, formula l_ seemed more probable since it explained more reasonably the mass 138.0423 (lOO%,$, 121.0347 (13%,!), 93.0476 (13%, C)]. spectrum [ m/e II COR 4 3 X=M R=OH R~=RFH L X=0 R=NH2 Rj=Rz=H z!