Three new macrocyclic trichothecenes, named 12'-hydroxyroridin E (1), roridin Q (2), and 2',3'deoxyroritoxin D (3), were isolated from the marine-derived fungus Myrothecium roridum TUF 98F42, and a new macrocyclic trichothecene, named roridin R (4), was isolated from Myrothecium sp. TUF 02F6 together with roridins A and H and isororidin E. The structures of new compounds were determined on the basis of their spectral data. Compound 2 possessed a unique ether moiety at the 13' position of 1. Compound 4 was a 2',3'-dihydro-2'-hydroxy derivative of roridin H. The IC50 values of compounds 1, 2, and 4 against the murine leukemia cell line L1210 were 0.19, 31.2, and 0.45 microM, respectively. Compound 3 showed antiyeast activity to Saccharomyces cerevisiae at 1 microg/disc (inhibition zone: 12.2 mm), which was about 10 time more active than roritoxin D (10.2 mm at 10 microg/disc).