Three new acylhomoserine lactone (AHL) inhibitors have been isolated from the Korean red alga, Ahnfeltiopsis flabelliformis, via bioactivity-guided fractionation using the recombinant Agrobacterium tumefaciens liquid culture bioassay. Unlike the majority of AHL inhibitors reported to date, these compounds were ${alpha}-D-galactopyranosyl-(1{ ightarrow}2)-glycerol$ (floridoside) (1), betonicine (2), and isethionic acid (3), all of which are structurally unrelated to AHLs.