Cyanobacterin is a secondary metabolite produced by the cyanobacterium, Scytonema hofmanni UTEX 2349, which inhibits the growth of other algae. The compound is a diaryl-substituted γ-lactone (Table 1). Cyanobacterin interrupts photosynthetic electron transport at a site in PS II. Analysis of PS II activity with various Hill electron acceptors suggests that the site of action is not identical to the classical PS II electron transport inhibitors (1).