We have previously shown that circular I/II-format hammerhead ribozymes have faster catalytic rates and a reduced divalent metal-ion requirement.(1,2) The enhanced-rate and reduced metal-ion dependence are an asset for in vivo use of hammerhead ribozymes as antisense therapeutics. However, circular hammerheads suffer two disadvantages. Their Km for substrate is increased relative to the wild-type hammerhead, and as Tm-format hammerheads they place greater constraints on the sequences that can be targeted, relative to I/III-format hammerheads. Here we show that these disadvantages can be eliminated in a novel hammerhead configuration, the lariat. The lariat is a I/III-format hammerhead that was constructed by combining chemical and enzymatic methods. The lariat hammerhead ribozyme possesses the same reduced magnesium-ion requirement and enhanced activity of our circular hammerheads;; Additionally, the lariat possesses no natural 5＇-or 3＇-termini and is therefore expected to be of greater resistance to nuclease degradation.