As reported previously, such inhibitors of glycosidases as panosialin, siastatin, pyridindolol and isoflavone rhamnosides have been found in culture filtrates of actinomycetes. Panosialin and siastatin inhibit sialidase, pyridindolol and isoflavone rhamnosides inhibit β-galactosidase. β-Galactosidase (EC 3.2.1.23) is a glycosidase that hydrolyses β-galactoside bond in glycoproteins, glycolipids and polysaccharides to yield terminal galactose, and is widely distributed among microorganisms, plants and animal tissues but its biological significance is still uncertain. In this communication, we describe the isolation and identification of p-hydroxyphenylacetaldoxime (HPAAO) which inhibits β-galactosidase. HPAAO was found in the culture filtrate of Streptomyces nigellus MD824-CG2 isolated from a soil sample collected in Saitama Prefecture, Japan, in 1973. Its structure was confirmed by comparison of IR, NMR, MS, TLC and activity with the synthetic one derived from tyramine. HPAAO is a potent inhibitor of β-galactosidase, did not show any inhibition against sialidase, showed no antibacterial and no antifungal activities at 100 μg/ml, and had low toxicity with no death caused by intraperitoneal injection of 500 mg/kg to mice.