From _Sturnira lilium parvidens_, with which it is sympatric, _S. l. occidentalis_ usually (but not always) differs in being brownish (rather than yellowish to yellowish orange) dorsally and in lacking epaulets, and differs in the following cranial features: first upper incisors simple (rather than weakly bifid in unworn condition), larger, and more nearly straight when viewed from the front; second upper incisors reduced; lower incisors bilobate rather than trilobate; lingual cusps on m1 and m2 greatly reduced; M2 usually turned inward from M1 at distinct angle. The two species have approximately the same external and cranial dimensions in western México.
_Measurements_ (in millimeters).--External measurements of the holotype are as follows: total length, 58; length of hind foot, 15; length of ear, 18; forearm (average of both), 42.5. Corresponding average and extreme measurements of 11 adults from 4 km. N Durazno, Jalisco, followed by those of eight adults from 17 km. SE Talpa, Jalisco, are: 61.9 (59-65), 60.9 (57-68); 14.1 (12-15), 13.0 (13); 16.1 (15-18), 16.0 (15-17); 42.2 (40.4-43.8), 42.9 (41.6-44.1); weight in grams, 16.8 (15-19, six specimens only), 19.2 (16.3-22.5).
Cranial measurements of the holotype additional to those given in Table 1 are: condyloincisive length, 19.7; breadth across upper canines, 5.5; length of mandibular tooth-row (c-m3), 6.7.
_Remarks._--The pattern of geographic variation in size in _Sturnira ludovici_ resembles that in many other species of tropical bats in North America in that individuals from the northern parts of the range are smaller than those from the south. Mexican specimens herein assigned to _S. l. ludovici_ average somewhat smaller than specimens from Central America and the northern part of South America (but are within the currently understood size limits of that subspecies) and average paler as well. Additional material is needed from central and eastern México before the limits of distribution of the two subspecies of _ludovici_ can be determined accurately.
All specimens examined of the new subspecies were trapped in mist nets. The holotype was captured in a net stretched across an old road among large fruit trees situated along a small river (a tributary of the Río del Baluarte). Tropical deciduous vegetation grew in the narrow valley of the river but the adjacent hills supported oak. A specimen of _Artibeus jamaicensis jamaicensis_ was netted along with the holotype and on the previous night, August 30, one individual each of _Glossophaga soricina leachii_ and _Sturnira lilium parvidens_ were taken in the same net. Baker and Greer (1962:69) also reported the two species of _Sturnira_ as netted together 6 mi. S Pueblo Nuevo in adjacent Durango.