**1. Introduction**

Fungal endophytes constitute an abundant and underexplored group of fungi that inhabit plants in diverse natural and human-managed ecosystems [1,2]. These symbiotic fungi often produce bioactive metabolites, some of which may improve the growth or resiliency of the host plant [3]. Recent studies have demonstrated that fungal endophytes are rich sources of small-molecule natural products with novel structures and biomedical potential [4,5]. In our continuing search for bioactive and/or novel metabolites from endosymbiotic microorganisms [6], we have investigated a cytotoxic extract of the fungal endophyte, *Teratosphaeria* sp. AK1128, isolated from a photosynthetic stem of *Equisetum arvense* (field horsetail, Equisetaceae). Herein, we report cytotoxicity assay-guided fractionation of this extract, resulting in isolation and characterization of ten metabolites, including three new naphtho-γ-pyrone dimers, teratopyrones A–C (**1**–**3**), and the constituent responsible for cytotoxic activity, nigerasperone A (**7**). *Teratosphaeria* is a genus within the newly established fungal family Teratosphaeriaceae (Dothideomycetes, Ascomycota), which has been distinguished recently from *Mycosphaerella* (Mycosphaerellaceae) [7]. To the best of our knowledge, this constitutes the second report on metabolites of a fungal strain of the family Teratosphaeriaceae [8].
