AbstractUnwashed rat liver microsomes were used to study the inhibition of the Δ6 and Δ9 desaturases by cyclopropenoid fatty acids with the ring structure about the 9,10 or 6,7 carbon atoms. The 9,10 cyclopropenoid acid (sterculic acid) is shown to be an effective inhibitor of only Δ9 desaturase and then only in the presence of MgCl2 and coenzyme A (prepresence of MgCl2 and coenzyme A (presumably due to the formation of sterculoyl‐CoA). Two 6,7 cyclopropenoid acids of different chain lengths showed no marked inhibition of either the Δ6 or Δ9 desaturase. By the use of [3H]‐sterculic acid, it has been shown that under conditions of high inhibition of the Δ9 desaturase the inhibitor is not covalently attached to the enzyme at any point. This disproves older ideas on the mechanism of inhibition that assumed reaction between the cyclopropenoid ring and sulphydryl groups on the enzymes.