Mirtrons are alternative precursors for microRNA biogenesis that were recently described in invertebrates. These short hairpin introns use splicing to bypass Drosha cleavage, which is otherwise essential for the generation of canonical animal microRNAs. Using computational and experimental strategies, we now establish that mammals have mirtrons as well. We identified 3 mirtrons that are well conserved and expressed in diverse mammals, 16 primate-specific mirtrons, and 46 candidates supported by limited cloning evidence in primates. As with some fly and worm mirtrons, the existence of well-conserved mammalian mirtrons indicates their relatively ancient incorporation into endogenous regulatory pathways. However, as worms, flies, and mammals each have different sets of mirtrons, we hypothesize that different animals may have independently evolved the capacity for this hybrid small RNA pathway. This notion is supported by our observation of several clade-specific features of mammalian and invertebrate mirtrons.