Papers on the biologically active substances contained in the mushroom Hericium erinaceum (Yamabushitake in Japanese) have been published. During the course of our screening program for pollen growth regulators among mushroom extracts, we found that H. erinaceum contains a new biologically active substance named hericerin (1). This paper describes the isolation, structural determination of 1, and its inhibitory effects on pine pollen germination and tea pollen growth. The artificially cultured mushroom (500 g fresh weight) was extracted with acetone, partitioned with EtOAc at pH 4, and purified via silica gel column chromatography (benzene-EtOAc, 9:1) and TLC (benzene-EtOH, 8:2), yielding pure hericerin as colorless needles (mp 138–140°C, 25 mg). The molecular formula C27H33NO3 was determined by HR-EIMS (obs. m/z 419.2425, M+) and elementary analysis. Structural elucidation using 1H-NMR, 13C-NMR, IR, and mass spectral data revealed 1 contains a (2'E)-geranyl group, a phenylethyl moiety, a methoxyl group, a phenolic hydroxyl group, and an isoindolinone nucleus. Hericerin inhibited pine pollen germination (≈25% at 30 mg/l, complete inhibition at 100 mg/l) and tea pollen growth (≈65% at 10 mg/l, ≈80% at 30 mg/l, marked inhibition at 100 mg/l).