<p>Fundulus herminiamatildae sp. nov. Is endemic from the Marmolejo stream, a head water tributary of Río Soto La Marina basin, in the municipality of Aramberri, Nuevo León, México. Geologically, it is located in the Northeastern province of México, specifically in the Sierra Madre Oriental subprovince. The water temperatura is a determinant and important factor for the species differentiation. Its closest relative is F. philpisteri. Fundulus herminiamatildae is distinguished from other killifishes by a large number of conspicuous and simple lateral bars, body with high profile, and the following proportions in cephalic length: snout (2.52.9, mean 2.7) eye diameter (4.1-4.9, mean 4.5), and Preorbital length (2.7-3.1, mean 2.9).&nbsp;</p>