The isolation and structural elucidation of ambiguine isonitriles A‒F from the terrestrial cyanophytes Fischerella ambigua UTEX 1903, Hapalosiphon hibernicus BZ-3-1, and Westiellopsis prolifica EN-3-1 are described. The structures of A‒F were determined by spectroscopic methods and confirmed for indolenine D by X-ray crystallography. The new alkaloids are characterized by an additional isoprene unit attached to C-2 of the indole moiety. In ambiguines D‒F the isoprenẏl substituent is further fused to the isonitrile-bearing carbon. © 1992, American Chemical Society. All rights reserved.