The skull and trigeminal jaw adductor musculature of the lizard families Gekkonidae, Pygopodidae and Xantusiidae are described. The external jaw adductor shows a different structure in the Gekkonidae and Pygopodidae than is observed in other lizards, approached only by the Xantusiidae and Feyliniidae. Paedomorphosis seems to be involved in the differentiation of the jaw adductor musculature in the Gekkonidae. The Gekkonidae and Pygopodidae may be hypothesized to form a monophyletic group, the Gekkota, on the basis of numerous synapomorphies. Within the Gekkota, the Pygopodidae are the sister-group of the Gekkonidae and retain some plesiomorphous features which are absent in the latter. The Xantusiidae share few synapomorphies with the Gekkota on the one hand, and some with scincomorph lizards on the other, especially with the Lacertidae.