In the spring of 1848, Phillip G. Ferguson wrote in his diary of having seen a number of “small fishes…many of them having glittering blue backs” in a warm spring a short distance south of El Carrizal in northern Chihuahua, for what we presume to be the first record of pupfishes (Cyprinodon) in that state or perhaps anywhere in Mexico. Here we discuss this, other 19^th-century, and subsequent occurrences of the genus in that region of North America, including our views on the species that Ferguson may have encountered, the scarcity of historical records of these fishes in Chihuahua, and possible reasons for the latter situation.