During the dry season (November through May) adult males apparently aestivate; several large series collected in the winter include only subadults and females. This absence of males is corroborated by personal observations in the Tepalcatepec Valley in April and May. In the summer rainy season the lizards are active in the morning and again in the late afternoon; only _Cnemidophorus calidipes_ is active during the heat of the midday. In some areas of the scrub forest _Cnemidophorus sacki zweifeli_ is found in association with _Cnemidophorus communis communis_. Throughout the scrub forest _C. sacki zweifeli_ occurs with _C. deppei infernalis_. In some of the more dense scrub forest, where _C. sacki zweifeli_ is not so abundant as in the more open forest, it has been taken with _C. lineatissimus exoristus_. In the open _Acacia-Cercidium_ associations on the valley floor _C. sacki zweifeli_ occurs with _C. calidipes_. This subspecies is not restricted to the scrub forest. On the lower slopes of the Cordillera Volcanica in Michoacán _C. sacki zweifeli_ has been collected in open pine-oak forest near Zirimicuaro and Ziracuaretiro. _Distribution._--_Cnemidophorus sacki zweifeli_ inhabits the valley of the Río Tepalcatepec in Michoacán and probably extreme southwestern Jalisco, and the western part of the Balsas Basin in Michoacán. No specimens have been seen from extreme western Guerrero, but _C. s. zweifeli_ may occur there. _Specimens examined._--Catalogue numbers are preceded by abbreviations of the name of the institution as listed in the acknowledgements. _Cnemidophorus sacki occidentalis_, 22 specimens, as follows: _Jalisco_: 8 km. E of Ameca, UMMZ 102045; 7 km. SE of Ameca, UMMZ 102046; Autlán, UMMZ 102044, 102219-21; 7 km. ESE of El Arenal, UMMZ 114736 (2); San Gabriel, UMMZ 102040, 102042-3. _Michoacán_: 2 km. ESE of Jiquilpan, UMMZ 117557 (3). _Nayarit_: Ixtlán del Río, UMMZ 104747; San José de la Conde, UMMZ 102047 (4); 5 km. N of Santa Isabel, UMMZ 102048-50. _Cnemidophorus sacki sacki_, 108 specimens, as follows: _Guerrero_: Chilpancingo, UMMZ 72426 (8), 73937 (7), 88422 (4); 8 km. W of Chilpancingo, UMMZ 119144 (4); 5 km. N of Iguala, UMMZ 114712 (11); 15 km. N of Iguala, UMMZ 99039; Mexcala, UMMZ 114711 (10); 8 km. S of Taxco, UMMZ 114709 (2). _Morelos_: Amacuzac, UMMZ 114716; 3 km. S of Cuautla, UMMZ 99031 (9), 99917 (11): Río Cuautla, UMMZ 99038 (6); 5 km. S of Temixco, UMMZ 114718 (12); _Puebla_: 5 km. SE of Izúcar de Matamoros, UMMZ 112650 (4); 13 km. SE of Izúcar de Matamoros, UMMZ 117497 (2); 3 km. NW of Tehuitzingo, UMMZ 114714 (10); 1 km. N of Teyuca, UMMZ 114713 (6).