Structured RNAs embedded in the untranslated regions (UTRs) of messenger RNAs can regulate gene expression. In bacteria, control of a metabolite gene is mediated by the self-cleaving activity of a ribozyme embedded in its 5' UTR. This discovery has raised the question of whether gene-regulating ribozymes also exist in eukaryotic mRNAs. Here we show that highly active hammerhead ribozymes are present in the 3' UTRs of rodent C-type lectin type II (Clec2) genes. Using a hammerhead RNA motif search with relaxed delimitation of the non-conserved regions, we detected ribozyme sequences in which the invariant regions, in contrast to the previously identified continuous hammerheads, occur as two fragments separated by hundreds of nucleotides. Notably, a fragment pair can assemble to form an active hammerhead ribozyme structure between the translation termination and the polyadenylation signals within the 3' UTR. We demonstrate that this hammerhead structure can self-cleave both in vitro and in vivo, and is able to reduce protein expression in mouse cells. These results indicate that an unrecognized mechanism of post-transcriptional gene regulation involving association of discontinuous ribozyme sequences within an mRNA may be modulating the expression of several CLEC2 proteins that function in bone remodelling and the immune response of several mammals.