Investigation of a microbial fermentation organic extract of Penicillium sp. H9318 led to the isolation of a new isoquinolinone alkaloid, (5S)-3,4,5,7-tetramethyl-5,8-dihydroxyl-6(5H)-isoquinolinone (1), along with four known citrinin derivatives (2-5). Citrinin (2) exhibited significant inhibitory activity against Streptomyces 85E in the hyphae formation inhibition (HFI) assay, while compounds 1, 3-5 were not active when tested at 20 mu g/disk in the HFI assay. Citrinin (2) further demonstrated a weak inhibitory activity against MCF-7 (IC50 71.93 mu mol L-1), LNCaP (IC50 77.92 mu mol L-1), LU-1 (147.85 mu mol L-1) and KB (IC50 65.93 mu mol L-1) cell lines, respectively, in the cytotoxicity assay.