Average and extreme external measurements of 13 adults (7 males, 6 females) from the vicinity of San Juan Ixcoy are: total length, 176.5 (168-187); length of tail, 105.8 (100-110); length of hind foot, 19.5 (19-20); length of ear from notch, 16.8 (15-18). The average weight in grams of these same specimens is 9.1 (8-11). Average and extreme cranial measurements of 12 specimens from the same series (7 males, 5 females) are: total length of skull, 22.55 (21.6-23.0); zygomatic breadth, 10.82 (10.2-11.2); breadth of braincase, 11.02 (10.6-11.4); depth of skull, 8.67 (8.4-8.9); interorbital constriction, 3.78 (3.6-3.9); breadth of rostrum, 3.86 (3.6-4.0); length of rostrum, 8.33 (8.1-8.6); length of incisive foramen, 4.27 (4.0-4.5); length of palate, 3.48 (3.2-3.8); length of upper molar tooth-row, 3.25 (3.0-3.4). ~_Reithrodontomys mexicanus howelli_~ Goodwin.--GUATEMALA.--Baja Verapaz: 5 mi. N, 1 mi. W [Santa Cruz] El Chol, 6000 ft., 3 (KU 65315, 65386-87--January 30, 1955). El Quiche: 1 mi. NE Nebaj, 6000 ft., 2 (KU 65275, 65277--December 19, 1954). Huehuetenango: 5 mi. E, 1 mi. N Huehuetenango, 7000 ft., 2 (KU 65418-19--December 22, 1954). Santa Rosa: 1 mi. WSW El Molino [= approximately 1 mi. S, 6 mi. E Cuajiniquilapa], 1 (KU 71315--August 3, 1956). The specimen from 1 mi. WSW El Molino is from within the range of the subspecies _R. m. orinus_ as mapped by Hooper (1952:141), coming from a locality to the southeast of the two westernmost localities shown by him for _orinus_. This mouse is distinctly darker than any of the specimens of _orinus_ discussed beyond, and is as dark as the darkest _howelli_ seen by us. In addition to the specimens of _howelli_ listed above, we have examined 12 from Prusia, Chiapas (UMMZ 88351-58, 96810-13), and three from 3 mi. NW San Cristóbal, Chiapas (KU 66680-82). _R. m. howelli_ may occur at moderate elevations along the coast of southern Guatemala, or possibly this specimen represents a population of dark-colored mice that is isolated from similarly colored populations to the north and west. According to the field notes of Albert A. Alcorn, the collector, this specimen was taken in a "tree set." Perhaps pale-colored and dark-colored populations of _R. mexicanus_ are sympatric but ecologically segregated in this region. ~_Reithrodontomys mexicanus lucifrons_~ Howell.--NICARAGUA.--Jinotega: 1 mi. NW Jinotega, 1 (KU 71344--April 12, 1956); 5 mi. S, 2 mi. E Jinotega, 1 (KU 71343--April 12, 1956). Comments on these specimens have appeared earlier (Englert, 1959:153). ~_Reithrodontomys mexicanus orinus_~ Hooper.--GUATEMALA.--Guatemala: 5 mi. S Guatemala City, 4950 ft., 2 (KU 65388-89--March 11, 1955); 6 mi. S Guatemala City, 4680 ft., 3 (KU 65390-92--March 10, 1955); 7 mi. S, 6 mi. E Guatemala City, 5800 ft., 2 (KU 65393-94--March 14, 1955).