Lolilline (1), a new indole−diterpenoid, was isolated from extracts of the seed of Lolium perenne infected with the endophytic fungus Acremonium lolii. Lolilline contained structural features present in both the lolitrem mycotoxins and the simpler indole−diterpenoid tremorgen paxilline, suggesting that lolilline may be an intermediate in the biosynthesis of the lolitrems from the presumed precursor paxilline. Although paxilline, terpendole C, and lolitrem B were found to be tremorgenic, lolilline was not, providing new insight into structure−activity relationships within the indole−diterpenoids. Keywords: Acremonium lolii; Lolium perenne; endophyte; lolitrem; paxilline; terpendole; tremor; ryegrass staggers; neurotoxin; mycotoxin; biosynthesis