The Senegalese sponge Microscleroderma spirophora has been found to pr
oduce exclusively very unusual 3 beta-methoxysteroids in place of the
common 3 beta-hydroxysteroids. Six different methoxysteroids (three of
which are novel compounds) have been isolated and identified by spect
roscopic means (MS, IR H-1 and C-13 NMR), and their taxonomic signific
ance discussed. A new method for the determination of the configuratio
n at C-24 in satuiated 24-ethyl side chains is also proposed.