Recent taxonomic works have recognized only two species of spiny pocket mice of the genus Heteromys (Rodentia: Heteromyidae) from Costa Rica. Within Costa Rica, the widespread H. desmarestianus is considered to occur throughout the wet Caribbean lowlands, as well as at middle and high elevations on the Caribbean and Pacific slopes of the country's main montane systems. In contrast, H. oresterus is known from only a few localities at high elevations in the western portion of the Cordillera de Talamanca in central Costa Rica. Our morphological and morphometric analyses of specimens from northwestern Costa Rica reveal the presence of an undescribed species of the genus, which we describe as Heteromys nubicolens. This new species ranges from 750 to 1840 m in elevation in the Cordillera de Tilarán and Cordillera de Guanacaste. Heteromys desmarestianus is found in the surrounding mesic lowlands and foothills. Externally, both species possess dark brown dorsal pelage, but H. nubicolens differs by overall larger size and by distinctive cranial proportions. In most cranial measurements, H. nubicolens is larger than H. desmarestianus; however, H. desmarestianus has a wider interorbital region and a wider braincase. Known populations of H. nubicolens occur in three highland areas (Monteverde, Volcán Rincón de la Vieja–Volcán Santa María, and Cerro Cacao), but populations in these areas are probably disjunct, being separated by intervening lowlands. Heteromys nubicolens is likely widespread throughout the Cordillera de Tilarán and Cordillera de Guanacaste, but its presence in other areas of the country is unlikely. Even after recognizing H. nubicolens as distinct from H. desmarestianus, morphological, karyological, and genetic data indicate that H. desmarestianus represents a species complex. Further research is therefore necessary to evaluate the taxonomic status of this species complex in other regions of the country and other parts of its widespread geographic distribution.

Recientes trabajos taxonómicos han considerado que sólo dos especies de ratones de abazones ( =  ratón bolsero, raton bolsón, ratón mochilero) del género Heteromys (Rodentia: Heteromyidae) habitan en Costa Rica. Heteromys desmarestianus, con una amplia distribución general, habita en ese país en las tierras bajas húmedas del Caribe, así como también en las estribaciones caribeñas y pacíficas de las principales cadenas montañosas, en altitudes medias y elevadas. Por el contrario, H. oresterus se conoce sólo de unas pocas localidades de alta montaña en la parte occidental de la Cordillera de Talamanca, en la región central de Costa Rica. Nuestros análisis morfológicos y morfométricos de ejemplares provenientes del noroccidente costarricense revelan la presencia de una especie no descrita del género, la cual describimos acá como Heteromys nubicolens. Esta nueva especie habita entre los 750 y los 1840 m de altitud en la Cordillera de Tilarán y la Cordillera de Guanacaste. Heteromys desmarestianus se encuentra en el piedemonte y en las tierras bajas húmedas adyacentes a estas cordilleras, rodeando la distribución de H. nubicolens. Externamente ambas especies poseen un pelaje dorsal de coloración parda oscura, pero