Abstract: Five new isocoumarins, paraphaeosphaerins A–C and chaetochiversins A and B, biogenetically related to monocillin I and radicicol, have been isolated from solid agar cultures of Paraphaeosphaeria quadriseptata and Chaetomium chiversii, two fungal strains living in association with the Sonoran desert plants, Opuntia leptocaulis and Ephedra fasciculata, respectively. A new chroman-4-one, aposphaerin C, was also isolated from P. quadriseptata. Their structures and stereochemistry were elucidated using a combination of 1H and 13C homo- and hetero-nuclear 2D NMR techniques, 1H NMR analysis of Mosher''s esters, and chemical correlations. [Copyright &y& Elsevier]