Momilactones A (1) and B (2), which have been identified as phytoalexins in rice, were isolated from extracts of the moss Hypnum plumaeforme. This is the first isolation and identification of momilactones as allelochemicals from a bryophyte. H. plumaeforme produces considerable amounts of momilactones (isolated yield: 8.4 mg/Kg plant for 1; 4.2 mg/Kg for 2). EtOAc extracts from H. plumaeforme and 2 showed growth inhibitory activity against angiosperms, moss, and liverwort plants. On the other hand, the growth of H. plumaeforme was insensitive to its extract and 2. Our finding suggests that momilactones play an important role as allelochemicals in this moss.