The mechanism underlying the determination of neurotransmitter phenotype in the developing mesencephalon, particularly GABAergic versus glutamatergic fate, remains largely unknown. Here, we show in mice that the basic helix-loop-helix transcriptional repressor gene Helt (also known as Megane and Heslike) functions as a selector gene that determines GABAergic over glutamatergic fate in the mesencephalon. Helt was coincidently expressed in all the progenitor domains for mesencephalic GABAergic neurons. In the mesencephalon of Helt-deficient embryos, GABAergic neurons were mostly absent and glutamatergic neurons emerged instead. Conversely, ectopically expressed Helt suppressed glutamatergic formation and induced GABAergic neurogenesis. However, the Helt mutants showed normal progenitor domain formation. In consequence, postmitotic expression of the homeodomain factor Nkx2.2, which was specifically expressed by GABAergic populations in wild-type embryos, was maintained despite the transmitter phenotype conversion from GABAergic to glutamatergic in the Helt mutants, suggesting that Helt is not involved in neuronal identity specification. Furthermore, we identified proneural genes Ngn1 and Ngn2, which were selectively expressed in glutamatergic progenitors in the developing mesencephalon and had the ability to confer the glutamatergic fate, as downstream target genes of Helt. These results suggest that Helt determines GABAergic over glutamatergic fate, at least in part, by repressing Ngn (Neurog) genes and that basic helix-loop-helix transcription factor networks involving Helt and Ngns are commonly used in the mesencephalon for determination of the GABAergic versus glutamatergic transmitter phenotype.