Marchantin A is a novel macrocyclic bis(bibenzyl)ether isolated from the liverwort Marchantia species. An X-ray study of its derivative, marchantin A trimethyl ether, revealed that the molecule possesses convex and concave surfaces, with a central hole on the concave surface. The centroid-centroid separations of opposing benzene rings are 8.80 and 4.55Å. A pharmacological study showed that the skeletal muscle relaxation activity is about 3.5 times less potent than that of d-tubocurarine. A comparison of the X-ray structure of marchantin A trimethyl ether and that of O, O', N-trimethyltubocurarine reported by Sobell et al. revealed that the molecules share almost the same macrocyclic bis(bibenzyl)ether skeleton structure, portions of which may therefore be crucial for the skeletal muscle relaxation activity.