The transcription factor Engrailed (En) controls the topography of axonal projections by regulating the expression of cell-adhesion molecules [1] [2] [3] [4] but it is not known whether it also controls the choice of individual synaptic target cells. In the cercal sensory system of the larval cockroach (Periplaneta americana), small numbers of identified wind-sensitive sensory neurons form highly specific synaptic connections with 14 identified giant interneurons [5] [6], and target-cell choice is independent of the pattern of axonal projections [6]. En is a putative positional determinant in the array of cercal sensory neurons [7]. In the present study, double-stranded RNA (dsRNA) interference [8] was used to abolish En expression. This treatment changed the axonal arborisation and synaptic outputs of an identified En-positive sensory neuron so that it came to resemble a nearby En-negative cell, which was itself unaffected. We thus demonstrate directly that En controls synaptic choice, as well as axon projections.