Aciculosporium and Heteroepichloë (Clavicipitaceae) are characteristic bambusicolous fungi in east Asia. In this study, we examined their intergeneric relationships based on the ALDH1-1 gene, which encodes a member of the aldehyde dehydrogenase family. In the clavicipitaceous fungi examined in this study, the nucleotide sequence of the third exon of ALDH1-1 (Exon-3) is 889 bp in length and has no insertion/deletion. A phylogenetic tree based on Exon-3 indicated that the clavicipitaceous fungi could be divided into two large groups: Cordyceps, Nomuraea, and Ustilaginoidea species formed a paraphyletic group, and the other grass biotrophic species formed a monophyletic group. This monophyletic group was further divided into three groups with high bootstrap support: i.e., species with Neotyphodium anamorphs (e.g., Epichloë), species with Ephelis anamorphs (e.g., Heteroepichloë), and Aciculosporium– Claviceps species. We discuss the relationships among Aciculosporium, Heteroepichloë, and other clavicipitaceous fungi.