A reinvestigation of the known marine natural product strobilinin I (R = H) has revealed it not to be a single compd., but to consist of two naturally occurring geometric isomers. (8E,13Z,20Z)- and (8Z,13E,20Z)-strobilinin I (R = Ac) were resolved, characterized, and their structures assigned by spectroscopic anal. It would appear that the abs. stereochem. of the strobilinins is very likely opposite to that of co-occurring variabilin .