The structure elucidation of emethallicins B (1) and C (2), novel epitetrathiodioxopiperazines possessing potent inhibitory activity of compound 48/80-induced histamine release from mast cells, isolated from the mycelial extract of Emericella heterothallica (mating type a), is reported based on their 1H and 13C n.m.r. spectra and a chemical correlation of emethallicin A (10) with apoaranotin (11); they are diesters of two C6-C2 acids having the same basic skeletons as apoaranotin (11) and aranotin (13), respectively. The IC50 values of these compounds were determined as 8.0 × 10-8 M for (1) and 1.0 × 10-6 M for (2).