Pseudellones A and B (1 and 2), a pair of irregularly bridged epimonothiodiketopiperazine diastereomers constructed from unusual 3-indolylglycine and alanine residues, and an alkaloid pseudellone C (3) possessing a unique skeleton were isolated from the marine-derived fungus Pseudallescheria ellipsoidea F42-3. Their structures were determined by spectroscopic data, ECD calculation, and X-ray single crystal diffraction. The biogenetic pathways of 1-3 were proposed, and 1H-indole-3-carboxylic acid (4), a plausible biosynthetic intermediate, was coisolated.