Shal K + (K v 4) channels in mammalian neurons have been shown to be localized exclusively to somato-dendritic regions of neurons, where they function as key determinants of dendritic excitability. To gain insight into the mechanisms underlying dendritic localization of K v 4 channels, we use Drosophila melanogaster as our model system. We show that Shal K + channels display a conserved somato-dendritic localization in vivo in Drosophila. From a yeast-2-hybrid screen, we identify the novel interactor, SIDL (for Shal Interactor of Di-Leucine Motif), as the first target protein reported to bind the highly conserved di-leucine motif (LL-motif) implicated in dendritic targeting. We show that SIDL is expressed primarily in the nervous system, co-localizes with GFP–Shal channels in neurons, and interacts specifically with the LL-motif of Drosophila and mouse Shal channels. We disrupt the Shal–SIDL interaction by mutating the LL-motif on Shal channels, and show that Shal K + channels are then mislocalized to some, but not all, axons in vivo. These results suggest that there are multiple mechanisms underlying Shal K + channel targeting, one of which depends on the LL-motif. The identification of SIDL may provide the first step for future investigation into the molecular machinery regulating the LL-motif-dependent targeting of K + channels.